diff --git a/src/cli.rs b/src/cli.rs
index 7d9f634..f3c5810 100644
--- a/src/cli.rs
+++ b/src/cli.rs
@@ -28,7 +28,7 @@ pub enum Error {
 
 pub enum Command {
     Parse { path: PathBuf },
-    Evaluate { path: PathBuf },
+    Run { path: PathBuf },
     Repl,
 }
 
@@ -43,9 +43,9 @@ impl Command {
                 let path = args.free_from_str()?;
                 Command::Parse { path }
             }
-            Some("evaluate") | Some("eval") => {
+            Some("run") => {
                 let path = args.free_from_str()?;
-                Command::Evaluate { path }
+                Command::Run { path }
             }
             Some("repl") | None => Command::Repl,
             Some(cmd) => return Err(Error::InvalidCommand(cmd.to_string())),
diff --git a/src/main.rs b/src/main.rs
index c4ad52a..5b628be 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,10 +1,13 @@
 use std::fs::read_to_string;
 use std::io::{stdin, stdout, BufRead, Write};
+use std::path::Path;
 
 use anyhow::{anyhow, Context, Result};
 
 use mul::cli::{Command, USAGE};
+use mul::phase::evaluate::Interpreter;
 use mul::phase::parse::{parse_interactive_entry, parse_program};
+use mul::syntax::Program;
 
 fn main() -> Result<()> {
     let Some(command) = Command::from_environment()? else {
@@ -13,24 +16,33 @@ fn main() -> Result<()> {
     };
     match command {
         Command::Parse { path } => {
-            let source = read_to_string(&path)
-                .with_context(|| format!("failed to open {}", path.display()))?;
-            let program = parse_program(source.as_str())
-                .map_err(|e| anyhow!("{e}"))
-                .with_context(|| format!("failed to parse {}", path.display()))?;
+            let program = parse(path.as_path())?;
             println!("{program:#?}");
             Ok(())
         }
-        Command::Evaluate { path } => {
-            todo!()
+        Command::Run { path } => {
+            let program = parse(path.as_path())?;
+            let mut interpreter = Interpreter::new();
+            let value = interpreter.run(&program)?;
+            println!("{value:#?}");
+            Ok(())
         }
         Command::Repl => repl(),
     }
 }
 
+fn parse(path: &Path) -> Result<Program> {
+    let source =
+        read_to_string(path).with_context(|| format!("failed to open {}", path.display()))?;
+    parse_program(source.as_str())
+        .map_err(|e| anyhow!("{e}"))
+        .with_context(|| format!("failed to parse {}", path.display()))
+}
+
 fn repl() -> Result<()> {
     let mut stdin = stdin().lock();
     let mut stdout = stdout().lock();
+    let mut interpreter = Interpreter::new();
     println!("welcome to M U L");
     loop {
         let mut line = String::new();
@@ -51,7 +63,8 @@ fn repl() -> Result<()> {
                 continue;
             }
         };
-        println!("{parsed:?}");
+        let value = interpreter.evaluate_interactive_entry(&parsed)?;
+        println!("{value:?}");
     }
     Ok(())
 }
diff --git a/src/phase/evaluate.rs b/src/phase/evaluate.rs
index 8b13789..808d2d6 100644
--- a/src/phase/evaluate.rs
+++ b/src/phase/evaluate.rs
@@ -1 +1,133 @@
+use std::collections::HashMap;
 
+use thiserror::Error;
+
+use crate::syntax::{Block, Expression, InteractiveEntry, Item, Name, Program};
+
+pub type Dictionary<T> = HashMap<Name, T>;
+
+#[derive(Debug, Clone)]
+pub enum Value {
+    Closure(Dictionary<Value>, Vec<Name>, Expression),
+    Boolean(bool),
+    Integer(i64),
+}
+
+#[derive(Debug, Error)]
+pub enum Error {
+    #[error("The name '{0} was not bound to any value in this context")]
+    UnboundName(Name),
+    #[error("Tried to call a term which was not a function")]
+    NotAFunction,
+    #[error("Mismatched number of function arguments and parameters: expected {0}, got {1}")]
+    ArgumentsMismatch(usize, usize),
+    #[error("No function called main to run")]
+    MissingMainFunction,
+}
+
+pub type Result<T> = std::result::Result<T, Error>;
+
+#[derive(Debug, Clone)]
+pub struct Interpreter {
+    environment: Dictionary<Value>,
+}
+
+impl Interpreter {
+    pub fn new() -> Self {
+        Interpreter {
+            environment: Dictionary::new(),
+        }
+    }
+
+    pub fn run(&mut self, program: &Program) -> Result<Value> {
+        for item in program {
+            self.interpret_item(item)?;
+        }
+        if let Some(main) = self.environment.get("main") {
+            // should main be an expression? a standalone closure type?
+            todo!()
+        } else {
+            Err(Error::MissingMainFunction)
+        }
+    }
+
+    pub fn interpret_item(&mut self, item: &Item) -> Result<Value> {
+        match item {
+            Item::Fn {
+                name,
+                parameters,
+                body,
+            } => {
+                let closure = Value::Closure(
+                    self.environment.clone(),
+                    parameters.clone(),
+                    Expression::Block(Box::new(body.clone())),
+                );
+                self.environment.insert(name.clone(), closure.clone());
+                Ok(closure)
+            }
+        }
+    }
+
+    pub fn evaluate_interactive_entry(&mut self, entry: &InteractiveEntry) -> Result<Value> {
+        match entry {
+            InteractiveEntry::Item(item) => self.interpret_item(item),
+            InteractiveEntry::Binding(binding) => {
+                self.bind(binding.name.clone(), &binding.expression)
+            }
+            InteractiveEntry::Expression(term) => self.evaluate(term),
+        }
+    }
+
+    pub fn evaluate(&mut self, term: &Expression) -> Result<Value> {
+        match term {
+            Expression::Variable(name) => self
+                .environment
+                .get(name)
+                .ok_or(Error::UnboundName(name.clone()))
+                .cloned(),
+            Expression::Block(b) => self.evaluate_block(b.as_ref()),
+            Expression::Lambda { parameters, result } => Ok(Value::Closure(
+                self.environment.clone(),
+                parameters.clone(),
+                *result.clone(),
+            )),
+            Expression::Call { callee, arguments } => match callee.as_ref() {
+                Expression::Lambda { parameters, result } => {
+                    if parameters.len() != arguments.len() {
+                        return Err(Error::ArgumentsMismatch(parameters.len(), arguments.len()));
+                    }
+                    let mut nested = self.clone();
+                    for (name, argument) in parameters.iter().zip(arguments) {
+                        // we don't want arguments to refer to each other, so use the parent interpreter
+                        nested.bind(name.clone(), argument)?;
+                    }
+                    nested.evaluate(result)
+                }
+                _ => Err(Error::NotAFunction),
+            },
+            Expression::Boolean(b) => Ok(Value::Boolean(*b)),
+            Expression::Integer(i) => Ok(Value::Integer(*i)),
+        }
+    }
+
+    fn evaluate_block(&mut self, block: &Block) -> Result<Value> {
+        let mut nested = self.clone();
+        for binding in &block.bindings {
+            nested.bind(binding.name.clone(), &binding.expression)?;
+        }
+        nested.evaluate(&block.result)
+    }
+
+    fn bind(&mut self, name: Name, term: &Expression) -> Result<Value> {
+        let value = self.evaluate(term)?;
+        self.environment.insert(name, value.clone());
+        Ok(value)
+    }
+}
+
+impl Default for Interpreter {
+    fn default() -> Self {
+        Self::new()
+    }
+}
diff --git a/src/phase/parse.rs b/src/phase/parse.rs
index 533878e..540668c 100644
--- a/src/phase/parse.rs
+++ b/src/phase/parse.rs
@@ -6,10 +6,10 @@ lalrpop_mod!(parser, "/phase/parse/parser.rs");
 
 pub type Result<'input, T> = std::result::Result<T, ParseError<usize, Token<'input>, &'static str>>;
 
-pub fn parse_program(input: &'_ str) -> Result<'_, Program> {
+pub fn parse_program(input: &str) -> Result<Program> {
     parser::ProgramParser::new().parse(input)
 }
 
-pub fn parse_interactive_entry(input: &'_ str) -> Result<'_, InteractiveEntry> {
+pub fn parse_interactive_entry(input: &str) -> Result<InteractiveEntry> {
     parser::InteractiveEntryParser::new().parse(input)
 }
diff --git a/src/phase/parse/parser.lalrpop b/src/phase/parse/parser.lalrpop
index 4aff32d..cffb5b7 100644
--- a/src/phase/parse/parser.lalrpop
+++ b/src/phase/parse/parser.lalrpop
@@ -14,7 +14,7 @@ pub Program: Program = Item*;
 
 pub InteractiveEntry: InteractiveEntry = {
     Item => InteractiveEntry::Item(<>),
-    Binding => InteractiveEntry::Binding(<>),
+    <Binding> ";"? => InteractiveEntry::Binding(<>),
     <Expression> ";"? => InteractiveEntry::Expression(<>),
 }
 
@@ -23,7 +23,7 @@ Item: Item = {
 }
 
 Binding: Binding =
-    "let" <Name> "=" <Expression> ";" => Binding(<>);
+    "let" <name:Name> "=" <expression:Expression> => Binding { <> };
 
 Expression: Expression = {
     Simple,
@@ -43,8 +43,10 @@ Atom: Expression = {
     "(" <Expression> ")",
 };
 
+BlockBinding = <Binding> ";";
+
 Block: Block =
-    "{" <bindings:Binding*> <result:Expression> "}" => Block { <> };
+    "{" <bindings:BlockBinding*> <result:Expression> "}" => Block { <> };
 
 // TODO: decide on identifier syntax
 Name: Name = r"[a-zA-Z_]+" => <>.to_string();
diff --git a/src/syntax.rs b/src/syntax.rs
index 1e05b7c..20cd2cf 100644
--- a/src/syntax.rs
+++ b/src/syntax.rs
@@ -26,7 +26,10 @@ pub enum Item {
 
 /// Syntactic mapping from an identifier to an expression.
 #[derive(Debug, Clone)]
-pub struct Binding(pub Name, pub Expression);
+pub struct Binding {
+    pub name: Name,
+    pub expression: Expression,
+}
 
 /// Some individually addressable syntax node, either atomic or compound.
 #[derive(Debug, Clone)]