diff --git a/src/eval.rs b/src/eval.rs
index 00a85f2..db97273 100644
--- a/src/eval.rs
+++ b/src/eval.rs
@@ -1,9 +1,11 @@
 pub mod normalise;
 pub mod const_fold;
 pub mod compose;
+pub mod differentiate;
 
-#[derive(Debug)]
+#[derive(Debug, PartialEq)]
 pub enum Error {
+	UnsupportedExpression,
 }
 
 pub trait Evaluator {
diff --git a/src/eval/differentiate.rs b/src/eval/differentiate.rs
new file mode 100644
index 0000000..41c3ca5
--- /dev/null
+++ b/src/eval/differentiate.rs
@@ -0,0 +1,208 @@
+use crate::expression::Expression;
+use crate::eval::{Error, Evaluator};
+
+pub struct Differentiator {
+	var: char,
+}
+
+impl Differentiator {
+	pub fn new(var: char) -> Self {
+		Differentiator { var }
+	}
+
+	fn diff(&self, expr: Expression) -> Result<Expression, Error> {
+		if expr.is_constant() {
+			Ok(Expression::Integer(0))
+		} else {
+			match expr {
+				Expression::Symbol(c) if c == self.var =>
+					Ok(Expression::Integer(1)),
+				Expression::Sum(x, y) => self.diff_sum(x, y),
+				Expression::Product(x, y) => self.diff_product(x, y),
+				Expression::Exponent(x, y) if y.is_constant() =>
+					self.diff_power(x, y),
+				_ => Err(Error::UnsupportedExpression),
+			}
+		}
+	}
+
+	fn diff_sum(
+		&self,
+		u: std::boxed::Box<Expression>,
+		v: std::boxed::Box<Expression>) -> Result<Expression, Error> {
+		let diff_u = self.diff_boxed(u)?;
+		let diff_v = self.diff_boxed(v)?;
+		Ok(Expression::Sum(diff_u, diff_v))
+	}
+
+	fn diff_product(
+		&self,
+		u: std::boxed::Box<Expression>,
+		v: std::boxed::Box<Expression>) -> Result<Expression, Error> {
+		let diff_u = self.diff_boxed(u.clone())?;
+		let diff_v = self.diff_boxed(v.clone())?;
+		Ok(Expression::Sum(
+			std::boxed::Box::new(Expression::Product(diff_u, v)),
+			std::boxed::Box::new(Expression::Product(u, diff_v))))
+	}
+
+	fn diff_power(
+		&self,
+		base: std::boxed::Box<Expression>,
+		power: std::boxed::Box<Expression>) -> Result<Expression, Error> {
+		let new_power = std::boxed::Box::new(
+			Expression::Difference(
+				power.clone(),
+				std::boxed::Box::new(Expression::Integer(1))));
+		let diff_base = self.diff_boxed(base.clone())?;
+		Ok(Expression::product(
+			Expression::Product(power, diff_base),
+			Expression::Exponent(base, new_power)))
+	}
+
+	fn diff_boxed(&self, mut expr: std::boxed::Box<Expression>)
+				  -> Result<std::boxed::Box<Expression>, Error> {
+		*expr = self.diff(*expr)?;
+		Ok(expr)
+	}
+}
+
+impl Evaluator for Differentiator {
+	type Expr = Expression;
+	type Error = Error;
+
+	fn eval(&mut self, input: Expression) -> Result<Expression, Error> {
+		self.diff(input)
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+
+	#[test]
+	fn derivative_of_integer_is_0() {
+		let input = Expression::Integer(1);
+		let result = Differentiator::new('x').eval(input).unwrap();
+		assert_eq!(result, Expression::Integer(0));
+	}
+
+	#[test]
+	fn derivative_of_half_is_zero() {
+		let input = Expression::quotient(
+			Expression::Integer(1),
+			Expression::Integer(2));
+		let result = Differentiator::new('x').eval(input).unwrap();
+		assert_eq!(result, Expression::Integer(0));
+	}
+
+	#[test]
+	fn derivative_of_x_is_1() {
+		let input = Expression::Symbol('x');
+		let result = Differentiator::new('x').eval(input).unwrap();
+		assert_eq!(result, Expression::Integer(1));
+	}
+
+	#[test]
+	fn derivative_of_sum_is_sum_of_derivatives() {
+		let input = Expression::sum(
+			Expression::Symbol('x'),
+			Expression::Integer(1));
+		let result = Differentiator::new('x').eval(input).unwrap();
+		assert_eq!(
+			result,
+			Expression::sum(
+				Expression::Integer(1),
+				Expression::Integer(0)));
+	}
+
+	#[test]
+	fn derivative_of_product_uses_product_rule() {
+		let input = Expression::product(
+			Expression::Integer(2),
+			Expression::Symbol('x'));
+		let result = Differentiator::new('x').eval(input).unwrap();
+		assert_eq!(
+			result,
+			Expression::sum(
+				Expression::product(
+					Expression::Integer(0),
+					Expression::Symbol('x')),
+				Expression::product(
+					Expression::Integer(2),
+					Expression::Integer(1))));
+	}
+
+	#[test]
+	fn derivative_of_exponent_of_x_uses_power_rule_and_chain_rule() {
+		let input = Expression::exponent(
+			Expression::Symbol('x'),
+			Expression::Integer(2));
+		let result = Differentiator::new('x').eval(input).unwrap();
+		assert_eq!(
+			result,
+			Expression::product(
+				Expression::product(
+					Expression::Integer(2),
+					Expression::Integer(1)),
+				Expression::exponent(
+					Expression::Symbol('x'),
+					Expression::difference(
+						Expression::Integer(2),
+						Expression::Integer(1)))));
+	}
+
+	#[test]
+	fn derivative_of_exponent_of_sum_of_exponent_uses_power_rule_and_chain_rule()
+	{
+		let input = Expression::exponent(
+			Expression::sum(
+				Expression::exponent(
+					Expression::Symbol('x'),
+					Expression::Integer(2)),
+				Expression::Integer(1)),
+			Expression::Integer(2));
+		let result = Differentiator::new('x').eval(input).unwrap();
+		assert_eq!(
+			result,
+			Expression::product(
+				Expression::product(
+					Expression::Integer(2),
+					Expression::sum(
+						Expression::product(
+							Expression::product(
+								Expression::Integer(2),
+								Expression::Integer(1)),
+							Expression::exponent(
+								Expression::Symbol('x'),
+								Expression::difference(
+									Expression::Integer(2),
+									Expression::Integer(1)))),
+						Expression::Integer(0))),
+				Expression::exponent(
+					Expression::sum(
+						Expression::exponent(
+							Expression::Symbol('x'),
+							Expression::Integer(2)),
+						Expression::Integer(1)),
+					Expression::difference(
+						Expression::Integer(2),
+						Expression::Integer(1)))));
+	}
+
+	#[test]
+	fn other_symbols_are_unsupported() {
+		let input = Expression::Symbol('y');
+		let result = Differentiator::new('x').eval(input).unwrap_err();
+		assert_eq!(result, Error::UnsupportedExpression);
+	}
+
+	#[test]
+	fn non_constant_exponents_are_unsupported() {
+		let input = Expression::exponent(
+			Expression::Symbol('x'),
+			Expression::Symbol('x'));
+		let result = Differentiator::new('x').eval(input).unwrap_err();
+		assert_eq!(result, Error::UnsupportedExpression);
+	}
+}
diff --git a/src/expression.rs b/src/expression.rs
index 7d58f86..e3187a6 100644
--- a/src/expression.rs
+++ b/src/expression.rs
@@ -1,4 +1,4 @@
-#[derive(Debug, PartialEq)]
+#[derive(Clone, Debug, PartialEq)]
 pub enum Expression {
 	Symbol(char),
 	Integer(i32),