diff --git a/examples/basic/cast1.ngr b/examples/basic/cast1.ngr
new file mode 100644
index 0000000..0dc2488
--- /dev/null
+++ b/examples/basic/cast1.ngr
@@ -0,0 +1,7 @@
+x8 = 5u8;
+x16 = <u16>x8 + 1u16;
+print x16;
+x32 = <u32>x16 + 1u32;
+print x32;
+x64 = <u64>x32 + 1u64;
+print x64;
\ No newline at end of file
diff --git a/examples/basic/cast2.ngr b/examples/basic/cast2.ngr
new file mode 100644
index 0000000..4c61255
--- /dev/null
+++ b/examples/basic/cast2.ngr
@@ -0,0 +1,7 @@
+x8 = 5i8;
+x16 = <i16>x8 - 1i16;
+print x16;
+x32 = <i32>x16 - 1i32;
+print x32;
+x64 = <i64>x32 - 1i64;
+print x64;
\ No newline at end of file
diff --git a/src/backend/error.rs b/src/backend/error.rs
index caa9e59..7509da3 100644
--- a/src/backend/error.rs
+++ b/src/backend/error.rs
@@ -1,4 +1,4 @@
-use crate::backend::runtime::RuntimeFunctionError;
+use crate::{backend::runtime::RuntimeFunctionError, eval::PrimitiveType, ir::Type};
 use codespan_reporting::diagnostic::Diagnostic;
 use cranelift_codegen::{isa::LookupError, settings::SetError, CodegenError};
 use cranelift_module::ModuleError;
@@ -39,6 +39,8 @@ pub enum BackendError {
     LookupError(#[from] LookupError),
     #[error(transparent)]
     Write(#[from] cranelift_object::object::write::Error),
+    #[error("Invalid type cast from {from} to {to}")]
+    InvalidTypeCast { from: PrimitiveType, to: Type },
 }
 
 impl From<BackendError> for Diagnostic<usize> {
@@ -64,6 +66,9 @@ impl From<BackendError> for Diagnostic<usize> {
             BackendError::Write(me) => {
                 Diagnostic::error().with_message(format!("Cranelift object write error: {}", me))
             }
+            BackendError::InvalidTypeCast { from, to } => Diagnostic::error().with_message(
+                format!("Internal error trying to cast from {} to {}", from, to),
+            ),
         }
     }
 }
@@ -103,6 +108,17 @@ impl PartialEq for BackendError {
                 BackendError::Write(b) => a == b,
                 _ => false,
             },
+
+            BackendError::InvalidTypeCast {
+                from: from1,
+                to: to1,
+            } => match other {
+                BackendError::InvalidTypeCast {
+                    from: from2,
+                    to: to2,
+                } => from1 == from2 && to1 == to2,
+                _ => false,
+            },
         }
     }
 }
diff --git a/src/backend/into_crane.rs b/src/backend/into_crane.rs
index 18f040f..d9e8701 100644
--- a/src/backend/into_crane.rs
+++ b/src/backend/into_crane.rs
@@ -1,6 +1,7 @@
 use std::collections::HashMap;
 
-use crate::ir::{Expression, Primitive, Program, Statement, Value, ValueOrRef};
+use crate::eval::PrimitiveType;
+use crate::ir::{Expression, Primitive, Program, Statement, Type, Value, ValueOrRef};
 use crate::syntax::ConstantType;
 use cranelift_codegen::entity::EntityRef;
 use cranelift_codegen::ir::{
@@ -307,6 +308,70 @@ impl Expression {
                 Err(BackendError::VariableLookupFailure(name))
             }
 
+            Expression::Cast(_, target_type, expr) => {
+                let (val, val_type) =
+                    expr.into_crane(builder, local_variables, global_variables)?;
+
+                match (val_type, &target_type) {
+                    (ConstantType::I8, Type::Primitive(PrimitiveType::I8)) => Ok((val, val_type)),
+                    (ConstantType::I8, Type::Primitive(PrimitiveType::I16)) => {
+                        Ok((builder.ins().sextend(types::I16, val), ConstantType::I16))
+                    }
+                    (ConstantType::I8, Type::Primitive(PrimitiveType::I32)) => {
+                        Ok((builder.ins().sextend(types::I32, val), ConstantType::I32))
+                    }
+                    (ConstantType::I8, Type::Primitive(PrimitiveType::I64)) => {
+                        Ok((builder.ins().sextend(types::I64, val), ConstantType::I64))
+                    }
+
+                    (ConstantType::I16, Type::Primitive(PrimitiveType::I16)) => Ok((val, val_type)),
+                    (ConstantType::I16, Type::Primitive(PrimitiveType::I32)) => {
+                        Ok((builder.ins().sextend(types::I32, val), ConstantType::I32))
+                    }
+                    (ConstantType::I16, Type::Primitive(PrimitiveType::I64)) => {
+                        Ok((builder.ins().sextend(types::I64, val), ConstantType::I64))
+                    }
+
+                    (ConstantType::I32, Type::Primitive(PrimitiveType::I32)) => Ok((val, val_type)),
+                    (ConstantType::I32, Type::Primitive(PrimitiveType::I64)) => {
+                        Ok((builder.ins().sextend(types::I64, val), ConstantType::I64))
+                    }
+
+                    (ConstantType::I64, Type::Primitive(PrimitiveType::I64)) => Ok((val, val_type)),
+
+                    (ConstantType::U8, Type::Primitive(PrimitiveType::U8)) => Ok((val, val_type)),
+                    (ConstantType::U8, Type::Primitive(PrimitiveType::U16)) => {
+                        Ok((builder.ins().uextend(types::I16, val), ConstantType::U16))
+                    }
+                    (ConstantType::U8, Type::Primitive(PrimitiveType::U32)) => {
+                        Ok((builder.ins().uextend(types::I32, val), ConstantType::U32))
+                    }
+                    (ConstantType::U8, Type::Primitive(PrimitiveType::U64)) => {
+                        Ok((builder.ins().uextend(types::I64, val), ConstantType::U64))
+                    }
+
+                    (ConstantType::U16, Type::Primitive(PrimitiveType::U16)) => Ok((val, val_type)),
+                    (ConstantType::U16, Type::Primitive(PrimitiveType::U32)) => {
+                        Ok((builder.ins().uextend(types::I32, val), ConstantType::U32))
+                    }
+                    (ConstantType::U16, Type::Primitive(PrimitiveType::U64)) => {
+                        Ok((builder.ins().uextend(types::I64, val), ConstantType::U64))
+                    }
+
+                    (ConstantType::U32, Type::Primitive(PrimitiveType::U32)) => Ok((val, val_type)),
+                    (ConstantType::U32, Type::Primitive(PrimitiveType::U64)) => {
+                        Ok((builder.ins().uextend(types::I64, val), ConstantType::U64))
+                    }
+
+                    (ConstantType::U64, Type::Primitive(PrimitiveType::U64)) => Ok((val, val_type)),
+
+                    _ => Err(BackendError::InvalidTypeCast {
+                        from: val_type.into(),
+                        to: target_type,
+                    }),
+                }
+            }
+
             Expression::Primitive(_, prim, mut vals) => {
                 let mut values = vec![];
                 let mut first_type = None;
diff --git a/src/bin/ngrc.rs b/src/bin/ngrc.rs
index 821b0e2..e486ec0 100644
--- a/src/bin/ngrc.rs
+++ b/src/bin/ngrc.rs
@@ -17,7 +17,7 @@ fn main() {
     let args = CommandLineArguments::parse();
     let mut compiler = ngr::Compiler::default();
 
-    let output_file = args.output.unwrap_or("output.o".to_string());
+    let output_file = args.output.unwrap_or_else(|| "output.o".to_string());
 
     if let Some(bytes) = compiler.compile(&args.file) {
         std::fs::write(&output_file, bytes)
diff --git a/src/eval.rs b/src/eval.rs
index cf77f74..a593a63 100644
--- a/src/eval.rs
+++ b/src/eval.rs
@@ -35,11 +35,13 @@
 //!     
 mod env;
 mod primop;
+mod primtype;
 mod value;
 
 use cranelift_module::ModuleError;
 pub use env::{EvalEnvironment, LookupError};
 pub use primop::PrimOpError;
+pub use primtype::PrimitiveType;
 pub use value::Value;
 
 use crate::backend::BackendError;
diff --git a/src/eval/primop.rs b/src/eval/primop.rs
index 5fa3a1c..c6bdc05 100644
--- a/src/eval/primop.rs
+++ b/src/eval/primop.rs
@@ -1,3 +1,4 @@
+use crate::eval::primtype::PrimitiveType;
 use crate::eval::value::Value;
 
 /// Errors that can occur running primitive operations in the evaluators.
@@ -22,6 +23,13 @@ pub enum PrimOpError {
     BadArgCount(String, usize),
     #[error("Unknown primitive operation {0}")]
     UnknownPrimOp(String),
+    #[error("Unsafe cast from {from} to {to}")]
+    UnsafeCast {
+        from: PrimitiveType,
+        to: PrimitiveType,
+    },
+    #[error("Unknown primitive type {0}")]
+    UnknownPrimType(String),
 }
 
 // Implementing primitives in an interpreter like this is *super* tedious,
diff --git a/src/eval/primtype.rs b/src/eval/primtype.rs
new file mode 100644
index 0000000..87559a1
--- /dev/null
+++ b/src/eval/primtype.rs
@@ -0,0 +1,121 @@
+use crate::{
+    eval::{PrimOpError, Value},
+    syntax::ConstantType,
+};
+use std::{fmt::Display, str::FromStr};
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum PrimitiveType {
+    U8,
+    U16,
+    U32,
+    U64,
+    I8,
+    I16,
+    I32,
+    I64,
+}
+
+impl Display for PrimitiveType {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            PrimitiveType::I8 => write!(f, "i8"),
+            PrimitiveType::I16 => write!(f, "i16"),
+            PrimitiveType::I32 => write!(f, "i32"),
+            PrimitiveType::I64 => write!(f, "i64"),
+            PrimitiveType::U8 => write!(f, "u8"),
+            PrimitiveType::U16 => write!(f, "u16"),
+            PrimitiveType::U32 => write!(f, "u32"),
+            PrimitiveType::U64 => write!(f, "u64"),
+        }
+    }
+}
+
+impl<'a> From<&'a Value> for PrimitiveType {
+    fn from(value: &Value) -> Self {
+        match value {
+            Value::I8(_) => PrimitiveType::I8,
+            Value::I16(_) => PrimitiveType::I16,
+            Value::I32(_) => PrimitiveType::I32,
+            Value::I64(_) => PrimitiveType::I64,
+            Value::U8(_) => PrimitiveType::U8,
+            Value::U16(_) => PrimitiveType::U16,
+            Value::U32(_) => PrimitiveType::U32,
+            Value::U64(_) => PrimitiveType::U64,
+        }
+    }
+}
+
+impl From<ConstantType> for PrimitiveType {
+    fn from(value: ConstantType) -> Self {
+        match value {
+            ConstantType::I8 => PrimitiveType::I8,
+            ConstantType::I16 => PrimitiveType::I16,
+            ConstantType::I32 => PrimitiveType::I32,
+            ConstantType::I64 => PrimitiveType::I64,
+            ConstantType::U8 => PrimitiveType::U8,
+            ConstantType::U16 => PrimitiveType::U16,
+            ConstantType::U32 => PrimitiveType::U32,
+            ConstantType::U64 => PrimitiveType::U64,
+        }
+    }
+}
+
+impl FromStr for PrimitiveType {
+    type Err = PrimOpError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s {
+            "i8" => Ok(PrimitiveType::I8),
+            "i16" => Ok(PrimitiveType::I16),
+            "i32" => Ok(PrimitiveType::I32),
+            "i64" => Ok(PrimitiveType::I64),
+            "u8" => Ok(PrimitiveType::U8),
+            "u16" => Ok(PrimitiveType::U16),
+            "u32" => Ok(PrimitiveType::U32),
+            "u64" => Ok(PrimitiveType::U64),
+            _ => Err(PrimOpError::UnknownPrimType(s.to_string())),
+        }
+    }
+}
+
+impl PrimitiveType {
+    /// Try to cast the given value to this type, returning the new value.
+    ///
+    /// Returns an error if the cast is not safe *in* *general*. This means that
+    /// this function will error even if the number will actually fit in the target
+    /// type, but it would not be generally safe to cast a member of the given
+    /// type to the target type. (So, for example, "1i64" is a number that could
+    /// work as a "u64", but since negative numbers wouldn't work, a cast from
+    /// "1i64" to "u64" will fail.)
+    pub fn safe_cast(&self, source: &Value) -> Result<Value, PrimOpError> {
+        match (self, source) {
+            (PrimitiveType::U8, Value::U8(x)) => Ok(Value::U8(*x)),
+            (PrimitiveType::U16, Value::U8(x)) => Ok(Value::U16(*x as u16)),
+            (PrimitiveType::U16, Value::U16(x)) => Ok(Value::U16(*x)),
+            (PrimitiveType::U32, Value::U8(x)) => Ok(Value::U32(*x as u32)),
+            (PrimitiveType::U32, Value::U16(x)) => Ok(Value::U32(*x as u32)),
+            (PrimitiveType::U32, Value::U32(x)) => Ok(Value::U32(*x)),
+            (PrimitiveType::U64, Value::U8(x)) => Ok(Value::U64(*x as u64)),
+            (PrimitiveType::U64, Value::U16(x)) => Ok(Value::U64(*x as u64)),
+            (PrimitiveType::U64, Value::U32(x)) => Ok(Value::U64(*x as u64)),
+            (PrimitiveType::U64, Value::U64(x)) => Ok(Value::U64(*x)),
+
+            (PrimitiveType::I8, Value::I8(x)) => Ok(Value::I8(*x)),
+            (PrimitiveType::I16, Value::I8(x)) => Ok(Value::I16(*x as i16)),
+            (PrimitiveType::I16, Value::I16(x)) => Ok(Value::I16(*x)),
+            (PrimitiveType::I32, Value::I8(x)) => Ok(Value::I32(*x as i32)),
+            (PrimitiveType::I32, Value::I16(x)) => Ok(Value::I32(*x as i32)),
+            (PrimitiveType::I32, Value::I32(x)) => Ok(Value::I32(*x)),
+            (PrimitiveType::I64, Value::I8(x)) => Ok(Value::I64(*x as i64)),
+            (PrimitiveType::I64, Value::I16(x)) => Ok(Value::I64(*x as i64)),
+            (PrimitiveType::I64, Value::I32(x)) => Ok(Value::I64(*x as i64)),
+            (PrimitiveType::I64, Value::I64(x)) => Ok(Value::I64(*x)),
+
+            _ => Err(PrimOpError::UnsafeCast {
+                from: source.into(),
+                to: *self,
+            }),
+        }
+    }
+}
diff --git a/src/ir/ast.rs b/src/ir/ast.rs
index e659b8d..4ce9ab2 100644
--- a/src/ir/ast.rs
+++ b/src/ir/ast.rs
@@ -1,10 +1,14 @@
-use crate::syntax::{self, ConstantType, Location};
+use crate::{
+    eval::PrimitiveType,
+    syntax::{self, ConstantType, Location},
+};
 use internment::ArcIntern;
 use pretty::{DocAllocator, Pretty};
 use proptest::{
     prelude::Arbitrary,
     strategy::{BoxedStrategy, Strategy},
 };
+use std::fmt;
 
 /// We're going to represent variables as interned strings.
 ///
@@ -115,6 +119,7 @@ where
 pub enum Expression {
     Value(Location, Value),
     Reference(Location, Variable),
+    Cast(Location, Type, ValueOrRef),
     Primitive(Location, Primitive, Vec<ValueOrRef>),
 }
 
@@ -127,6 +132,11 @@ where
         match self {
             Expression::Value(_, val) => val.pretty(allocator),
             Expression::Reference(_, var) => allocator.text(var.as_ref().to_string()),
+            Expression::Cast(_, t, e) => allocator
+                .text("<")
+                .append(t.pretty(allocator))
+                .append(allocator.text(">"))
+                .append(e.pretty(allocator)),
             Expression::Primitive(_, op, exprs) if exprs.len() == 1 => {
                 op.pretty(allocator).append(exprs[0].pretty(allocator))
             }
@@ -283,3 +293,28 @@ where
         }
     }
 }
+
+#[derive(Debug, Eq, PartialEq)]
+pub enum Type {
+    Primitive(PrimitiveType),
+}
+
+impl<'a, 'b, D, A> Pretty<'a, D, A> for &'b Type
+where
+    A: 'a,
+    D: ?Sized + DocAllocator<'a, A>,
+{
+    fn pretty(self, allocator: &'a D) -> pretty::DocBuilder<'a, D, A> {
+        match self {
+            Type::Primitive(pt) => allocator.text(format!("{}", pt)),
+        }
+    }
+}
+
+impl fmt::Display for Type {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Type::Primitive(pt) => pt.fmt(f),
+        }
+    }
+}
diff --git a/src/ir/eval.rs b/src/ir/eval.rs
index 4830ce3..8d46368 100644
--- a/src/ir/eval.rs
+++ b/src/ir/eval.rs
@@ -1,7 +1,7 @@
 use crate::eval::{EvalEnvironment, EvalError, Value};
 use crate::ir::{Expression, Program, Statement};
 
-use super::{Primitive, ValueOrRef};
+use super::{Primitive, Type, ValueOrRef};
 
 impl Program {
     /// Evaluate the program, returning either an error or a string containing everything
@@ -47,6 +47,19 @@ impl Expression {
 
             Expression::Reference(_, n) => Ok(env.lookup(n.clone())?),
 
+            Expression::Cast(_, t, valref) => {
+                let value = match valref {
+                    ValueOrRef::Ref(_, n) => env.lookup(n.clone())?,
+                    ValueOrRef::Value(loc, val) => {
+                        Expression::Value(loc.clone(), val.clone()).eval(env)?
+                    }
+                };
+
+                match t {
+                    Type::Primitive(pt) => Ok(pt.safe_cast(&value)?),
+                }
+            }
+
             Expression::Primitive(_, op, args) => {
                 let mut arg_values = Vec::with_capacity(args.len());
 
diff --git a/src/ir/from_syntax.rs b/src/ir/from_syntax.rs
index 88d0813..3f174c6 100644
--- a/src/ir/from_syntax.rs
+++ b/src/ir/from_syntax.rs
@@ -1,6 +1,8 @@
 use internment::ArcIntern;
+use std::str::FromStr;
 use std::sync::atomic::AtomicUsize;
 
+use crate::eval::PrimitiveType;
 use crate::ir::ast as ir;
 use crate::syntax;
 
@@ -104,7 +106,20 @@ impl syntax::Expression {
                 (vec![], ValueOrRef::Ref(loc, ArcIntern::new(name)))
             }
 
-            syntax::Expression::Cast(_, _, _) => unimplemented!(),
+            syntax::Expression::Cast(loc, t, expr) => {
+                let (mut prereqs, new_expr) = expr.rebind(base_name);
+                let new_name = gensym(base_name);
+                prereqs.push(ir::Statement::Binding(
+                    loc.clone(),
+                    new_name.clone(),
+                    ir::Expression::Cast(
+                        loc.clone(),
+                        ir::Type::Primitive(PrimitiveType::from_str(&t).unwrap()),
+                        new_expr,
+                    ),
+                ));
+                (prereqs, ValueOrRef::Ref(loc, new_name))
+            }
 
             // Primitive expressions are where we do the real work.
             syntax::Expression::Primitive(loc, prim, mut expressions) => {
diff --git a/src/syntax/arbitrary.rs b/src/syntax/arbitrary.rs
index 51cec90..1fd9b8d 100644
--- a/src/syntax/arbitrary.rs
+++ b/src/syntax/arbitrary.rs
@@ -124,7 +124,30 @@ impl Arbitrary for Expression {
             Union::new([value_strategy, reference_strategy]).boxed()
         };
 
-        leaf_strategy
+        let cast_strategy = if let Some(bigger_type) = target_type {
+            let mut smaller_types = bigger_type.safe_casts_to();
+
+            if smaller_types.is_empty() {
+                leaf_strategy
+            } else {
+                let duplicated_env = defined_variables.clone();
+                let cast_exp = |t, e| Expression::Cast(Location::manufactured(), t, Box::new(e));
+
+                let smaller_strats: Vec<BoxedStrategy<Expression>> = smaller_types
+                    .drain(..)
+                    .map(|t| {
+                        Expression::arbitrary_with((Some(duplicated_env.clone()), Some(t)))
+                            .prop_map(move |e| cast_exp(t.name(), e))
+                            .boxed()
+                    })
+                    .collect();
+                Union::new(smaller_strats).boxed()
+            }
+        } else {
+            leaf_strategy
+        };
+
+        cast_strategy
             .prop_recursive(3, 64, 2, move |inner| {
                 (select(OPERATORS), proptest::collection::vec(inner, 2)).prop_map(
                     move |((operator, arg_count), mut exprs)| {
diff --git a/src/syntax/eval.rs b/src/syntax/eval.rs
index 9122dd9..f952e45 100644
--- a/src/syntax/eval.rs
+++ b/src/syntax/eval.rs
@@ -1,7 +1,8 @@
 use internment::ArcIntern;
 
-use crate::eval::{EvalEnvironment, EvalError, Value};
+use crate::eval::{EvalEnvironment, EvalError, PrimitiveType, Value};
 use crate::syntax::{ConstantType, Expression, Program, Statement};
+use std::str::FromStr;
 
 impl Program {
     /// Evaluate the program, returning either an error or what it prints out when run.
@@ -59,7 +60,11 @@ impl Expression {
 
             Expression::Reference(_, n) => Ok(env.lookup(ArcIntern::new(n.clone()))?),
 
-            Expression::Cast(_, _, _) => unimplemented!(),
+            Expression::Cast(_, target, expr) => {
+                let target_type = PrimitiveType::from_str(target)?;
+                let value = expr.eval(env)?;
+                Ok(target_type.safe_cast(&value)?)
+            }
 
             Expression::Primitive(_, op, args) => {
                 let mut arg_values = Vec::with_capacity(args.len());
diff --git a/src/syntax/tokens.rs b/src/syntax/tokens.rs
index 682e266..a545435 100644
--- a/src/syntax/tokens.rs
+++ b/src/syntax/tokens.rs
@@ -175,6 +175,34 @@ impl ConstantType {
             ConstantType::I8 | ConstantType::I16 | ConstantType::I32 | ConstantType::I64
         )
     }
+
+    /// Return the set of types that can be safely casted into this type.
+    pub fn safe_casts_to(self) -> Vec<ConstantType> {
+        match self {
+            ConstantType::I8 => vec![],
+            ConstantType::I16 => vec![ConstantType::I8],
+            ConstantType::I32 => vec![ConstantType::I16, ConstantType::I8],
+            ConstantType::I64 => vec![ConstantType::I32, ConstantType::I16, ConstantType::I8],
+            ConstantType::U8 => vec![],
+            ConstantType::U16 => vec![ConstantType::U8],
+            ConstantType::U32 => vec![ConstantType::U16, ConstantType::U8],
+            ConstantType::U64 => vec![ConstantType::U32, ConstantType::U16, ConstantType::U8],
+        }
+    }
+
+    /// Return the name of the given type, as a string
+    pub fn name(&self) -> String {
+        match self {
+            ConstantType::I8 => "i8".to_string(),
+            ConstantType::I16 => "i16".to_string(),
+            ConstantType::I32 => "i32".to_string(),
+            ConstantType::I64 => "i64".to_string(),
+            ConstantType::U8 => "u8".to_string(),
+            ConstantType::U16 => "u16".to_string(),
+            ConstantType::U32 => "u32".to_string(),
+            ConstantType::U64 => "u64".to_string(),
+        }
+    }
 }
 
 #[derive(Debug, Error, PartialEq)]
diff --git a/src/syntax/validate.rs b/src/syntax/validate.rs
index feb4ea4..676ff88 100644
--- a/src/syntax/validate.rs
+++ b/src/syntax/validate.rs
@@ -127,7 +127,7 @@ impl Expression {
                 vec![Error::UnboundVariable(loc.clone(), var.clone())],
                 vec![],
             ),
-            Expression::Cast(_, _, _) => unimplemented!(),
+            Expression::Cast(_, _, expr) => expr.validate(variable_map),
             Expression::Primitive(_, _, args) => {
                 let mut errors = vec![];
                 let mut warnings = vec![];