diff --git a/src/env.rs b/src/env.rs
deleted file mode 100644
index 53ae9d1..0000000
--- a/src/env.rs
+++ /dev/null
@@ -1,123 +0,0 @@
-use std::{borrow::Borrow, collections::HashMap, rc::Rc};
-
-use crate::eval::{Error, Expression};
-
-pub type Environment = HashMap<String, Rc<Expression>>;
-
-pub fn core_environment() -> Environment {
-    [
-        // Arithmetic operations
-        (
-            "+".to_string(),
-            Expression::NativeFunc {
-                func: |args| {
-                    let res = args
-                        .into_iter()
-                        .reduce(|lhs, rhs| match (lhs.borrow(), rhs.borrow()) {
-                            (Expression::Int(lhs), Expression::Int(rhs)) => {
-                                Expression::Int(lhs + rhs).into()
-                            }
-                            _ => todo!(),
-                        })
-                        .unwrap_or(Rc::new(Expression::Int(0)));
-
-                    Ok(res)
-                },
-            }
-            .into(),
-        ),
-        (
-            "-".to_string(),
-            Expression::NativeFunc {
-                func: |args| {
-                    let res = args
-                        .into_iter()
-                        .reduce(|lhs, rhs| match (lhs.borrow(), rhs.borrow()) {
-                            (Expression::Int(lhs), Expression::Int(rhs)) => {
-                                Expression::Int(lhs - rhs).into()
-                            }
-                            _ => todo!(),
-                        })
-                        .unwrap_or(Rc::new(Expression::Int(0)));
-
-                    Ok(res)
-                },
-            }
-            .into(),
-        ),
-        (
-            "*".to_string(),
-            Expression::NativeFunc {
-                func: |args| {
-                    let res = args
-                        .into_iter()
-                        .reduce(|lhs, rhs| match (lhs.borrow(), rhs.borrow()) {
-                            (Expression::Int(lhs), Expression::Int(rhs)) => {
-                                Expression::Int(lhs * rhs).into()
-                            }
-                            _ => todo!(),
-                        })
-                        .unwrap_or(Rc::new(Expression::Int(0)));
-
-                    Ok(res)
-                },
-            }
-            .into(),
-        ),
-        (
-            "/".to_string(),
-            Expression::NativeFunc {
-                func: |args| {
-                    let res = args
-                        .into_iter()
-                        .reduce(|lhs, rhs| match (lhs.borrow(), rhs.borrow()) {
-                            (Expression::Int(lhs), Expression::Int(rhs)) => {
-                                Expression::Int(lhs / rhs).into()
-                            }
-                            _ => todo!(),
-                        })
-                        .unwrap_or(Rc::new(Expression::Int(0)));
-
-                    Ok(res)
-                },
-            }
-            .into(),
-        ),
-        // Collections
-        (
-            "vector".to_string(),
-            Expression::NativeFunc {
-                func: |args| Ok(Rc::new(Expression::Vector(args))),
-            }
-            .into(),
-        ),
-        (
-            "hashmap".to_string(),
-            Expression::NativeFunc {
-                func: |args| {
-                    if args.len() % 2 != 0 {
-                        Err(Error::MismatchedArgCount)?
-                    }
-
-                    let mut index = -1;
-                    let (keys, values): (Vec<_>, Vec<_>) = args.into_iter().partition(|_| {
-                        index += 1;
-                        index % 2 == 0
-                    });
-
-                    let res = keys
-                        .into_iter()
-                        // We turn the keys into strings because they're hashable
-                        // This feels so hacky, but ¯\_(ツ)_/¯
-                        .map(|key| key.to_string())
-                        .zip(values.into_iter())
-                        .collect();
-
-                    Ok(Rc::new(Expression::HashMap(res)))
-                },
-            }
-            .into(),
-        ),
-    ]
-    .into()
-}
diff --git a/src/evaluator/env.rs b/src/evaluator/env.rs
new file mode 100644
index 0000000..8e09a63
--- /dev/null
+++ b/src/evaluator/env.rs
@@ -0,0 +1,186 @@
+use std::{borrow::Borrow, cell::RefCell, collections::HashMap, rc::Rc};
+
+use super::{eval_ast_node, Error, Expression};
+use crate::parser::Node;
+
+pub type RawEnvironment = HashMap<String, Rc<Expression>>;
+pub struct Environment {
+    current: RefCell<RawEnvironment>,
+    outer: Option<Rc<Environment>>,
+}
+
+impl Environment {
+    pub fn get(&self, ident: &str) -> Option<Rc<Expression>> {
+        if let Some(val) = self.current.borrow().get(ident) {
+            return Some(val.clone());
+        }
+
+        self.outer.clone()?.get(ident)
+    }
+
+    fn set(&self, ident: String, val: Rc<Expression>) {
+        self.current.borrow_mut().insert(ident, val);
+    }
+}
+
+pub fn core_environment() -> Rc<Environment> {
+    let env = [
+        // Arithmetic operations
+        (
+            "+".to_string(),
+            Expression::NativeFunc(|args| {
+                let res = args
+                    .into_iter()
+                    .reduce(|lhs, rhs| match (lhs.borrow(), rhs.borrow()) {
+                        (Expression::Int(lhs), Expression::Int(rhs)) => {
+                            Expression::Int(lhs + rhs).into()
+                        }
+                        _ => todo!(),
+                    })
+                    .unwrap_or(Rc::new(Expression::Int(0)));
+
+                Ok(res)
+            })
+            .into(),
+        ),
+        (
+            "-".to_string(),
+            Expression::NativeFunc(|args| {
+                let res = args
+                    .into_iter()
+                    .reduce(|lhs, rhs| match (lhs.borrow(), rhs.borrow()) {
+                        (Expression::Int(lhs), Expression::Int(rhs)) => {
+                            Expression::Int(lhs - rhs).into()
+                        }
+                        _ => todo!(),
+                    })
+                    .unwrap_or(Rc::new(Expression::Int(0)));
+
+                Ok(res)
+            })
+            .into(),
+        ),
+        (
+            "*".to_string(),
+            Expression::NativeFunc(|args| {
+                let res = args
+                    .into_iter()
+                    .reduce(|lhs, rhs| match (lhs.borrow(), rhs.borrow()) {
+                        (Expression::Int(lhs), Expression::Int(rhs)) => {
+                            Expression::Int(lhs * rhs).into()
+                        }
+                        _ => todo!(),
+                    })
+                    .unwrap_or(Rc::new(Expression::Int(0)));
+
+                Ok(res)
+            })
+            .into(),
+        ),
+        (
+            "/".to_string(),
+            Expression::NativeFunc(|args| {
+                let res = args
+                    .into_iter()
+                    .reduce(|lhs, rhs| match (lhs.borrow(), rhs.borrow()) {
+                        (Expression::Int(lhs), Expression::Int(rhs)) => {
+                            Expression::Int(lhs / rhs).into()
+                        }
+                        _ => todo!(),
+                    })
+                    .unwrap_or(Rc::new(Expression::Int(0)));
+
+                Ok(res)
+            })
+            .into(),
+        ),
+        // Collections
+        (
+            "vector".to_string(),
+            Expression::NativeFunc(|args| Ok(Rc::new(Expression::Vector(args)))).into(),
+        ),
+        (
+            "hashmap".to_string(),
+            Expression::NativeFunc(|args| {
+                if args.len() % 2 != 0 {
+                    Err(Error::MismatchedArgCount)?
+                }
+
+                let mut index = -1;
+                let (keys, values): (Vec<_>, Vec<_>) = args.into_iter().partition(|_| {
+                    index += 1;
+                    index % 2 == 0
+                });
+
+                let res = keys
+                    .into_iter()
+                    // We turn the keys into strings because they're hashable
+                    // This feels so hacky, but ¯\_(ツ)_/¯
+                    .map(|key| key.to_string())
+                    .zip(values.into_iter())
+                    .collect();
+
+                Ok(Rc::new(Expression::HashMap(res)))
+            })
+            .into(),
+        ),
+        // Environment Manipulation
+        (
+            "define!".to_string(),
+            Expression::Special(|env, args| {
+                let mut args = args.into_iter();
+                if args.len() != 2 {
+                    Err(Error::MismatchedArgCount)?
+                }
+
+                let key = match args.next().unwrap() {
+                    Node::Symbol(name) => name,
+                    _ => Err(Error::ExpectedSymbol)?,
+                };
+
+                let val = eval_ast_node(env.clone(), args.next().unwrap())?;
+
+                env.set(key, val.clone());
+
+                Ok(val)
+            })
+            .into(),
+        ),
+        (
+            "let*".to_string(),
+            Expression::Special(|env, args| {
+                if args.len() != 2 {
+                    Err(Error::MismatchedArgCount)?
+                }
+
+                let mut args = args.into_iter();
+                let new_env = match args.next().unwrap() {
+                    Node::List(list) if list.len() == 2 => {
+                        let mut list = list;
+                        let sym = match list.remove(0) {
+                            Node::Symbol(name) => name,
+                            _ => Err(Error::ExpectedSymbol)?,
+                        };
+
+                        let val = eval_ast_node(env.clone(), list.remove(0))?;
+                        Environment {
+                            current: RefCell::new([(sym.clone(), val)].into()),
+                            outer: Some(env.clone()),
+                        }
+                    }
+                    Node::List(_) => Err(Error::MismatchedArgCount)?,
+                    _ => Err(Error::ExpectedList)?,
+                };
+
+                eval_ast_node(Rc::new(new_env), args.next().unwrap())
+            })
+            .into(),
+        ),
+    ];
+
+    Environment {
+        current: RefCell::new(env.into()),
+        outer: None,
+    }
+    .into()
+}
diff --git a/src/eval.rs b/src/evaluator/mod.rs
similarity index 72%
rename from src/eval.rs
rename to src/evaluator/mod.rs
index 04a53b7..603b724 100644
--- a/src/eval.rs
+++ b/src/evaluator/mod.rs
@@ -3,7 +3,10 @@ use std::{borrow::Borrow, collections::HashMap, rc::Rc};
 use anyhow::Result;
 use thiserror::Error;
 
-use crate::{env::Environment, parser::Node};
+use crate::parser::Node;
+
+mod env;
+pub use env::{core_environment, Environment};
 
 thread_local! {
     static TRUE: Rc<Expression> = Rc::new(Expression::Boolean(true));
@@ -24,9 +27,8 @@ pub enum Expression {
     Vector(Vec<Rc<Expression>>),
     HashMap(HashMap<String, Rc<Expression>>),
 
-    NativeFunc {
-        func: fn(args: Vec<Rc<Expression>>) -> Result<Rc<Expression>>,
-    },
+    NativeFunc(fn(args: Vec<Rc<Expression>>) -> Result<Rc<Expression>>),
+    Special(fn(env: Rc<Environment>, args: Vec<Node>) -> Result<Rc<Expression>>),
 }
 
 impl std::fmt::Display for Expression {
@@ -58,7 +60,8 @@ impl std::fmt::Display for Expression {
                 write!(f, "{{{res}}}")
             }
 
-            Expression::NativeFunc { .. } => write!(f, "function"),
+            Expression::NativeFunc(func) => write!(f, "{func:?}"),
+            Expression::Special(func) => write!(f, "{func:?}"),
         }
     }
 }
@@ -69,38 +72,45 @@ pub enum Error {
     NotInEnv,
     #[error("expression does not have a valid operator")]
     InvalidOperator,
+    #[error("expected symbol")]
+    ExpectedSymbol,
+    #[error("expected list")]
+    ExpectedList,
     #[error("incorrect number of arguments passed to function")]
     MismatchedArgCount,
 }
 
-pub fn eval(env: &Environment, ast: Vec<Node>) -> Result<Vec<Rc<Expression>>> {
+pub fn eval(env: Rc<Environment>, ast: Vec<Node>) -> Result<Vec<Rc<Expression>>> {
     let mut res = Vec::new();
 
     for node in ast {
-        res.push(eval_ast_node(env, node)?);
+        res.push(eval_ast_node(env.clone(), node)?);
     }
 
     Ok(res)
 }
 
-fn eval_ast_node(env: &Environment, ast_node: Node) -> Result<Rc<Expression>> {
+fn eval_ast_node(env: Rc<Environment>, ast_node: Node) -> Result<Rc<Expression>> {
     let expr = match ast_node {
         Node::List(list) => {
-            let mut res = Vec::new();
-            for node in list {
-                res.push(eval_ast_node(env, node)?);
+            let mut list = list.into_iter();
+            let operator = eval_ast_node(env.clone(), list.next().ok_or(Error::InvalidOperator)?)?;
+
+            match operator.borrow() {
+                Expression::NativeFunc(func) => {
+                    let mut args = Vec::new();
+                    for node in list {
+                        args.push(eval_ast_node(env.clone(), node)?);
+                    }
+
+                    func(args)?
+                }
+                Expression::Special(func) => {
+                    let args = list.collect();
+                    func(env, args)?
+                }
+                _ => Err(Error::InvalidOperator)?,
             }
-
-            let mut list = res.into_iter();
-            let operator = match list.next() {
-                Some(rc) => match rc.borrow() {
-                    Expression::NativeFunc { func } => *func,
-                    _ => Err(Error::InvalidOperator)?,
-                },
-                None => Err(Error::InvalidOperator)?,
-            };
-
-            operator(list.collect())?
         }
         Node::Symbol(sym) => env.get(&sym).ok_or(Error::NotInEnv)?.to_owned(),
 
@@ -118,7 +128,6 @@ fn eval_ast_node(env: &Environment, ast_node: Node) -> Result<Rc<Expression>> {
 
 #[cfg(test)]
 mod test {
-    use crate::core_environment;
     use crate::lexer;
     use crate::parser;
 
@@ -149,8 +158,11 @@ mod test {
     #[case("[1 (+ 1 2)]", "[1 3]")]
     #[case("{}", "{}")]
     #[case("{:a \"uwu\"}", "{:a: uwu}")]
+    // Environment manipulation
+    #[case("(define! asdf (+ 2 2)) (+ asdf 2)", "4\n6")]
+    #[case("(let* (a 2) (+ a 2))", "4")]
     fn test_evaluator(#[case] input: &str, #[case] expected: &str) {
-        let env = &core_environment();
+        let env = core_environment();
         let tokens = lexer::read(input).unwrap();
         let ast = parser::parse(tokens).unwrap();
         let res = eval(env, ast)
@@ -167,7 +179,7 @@ mod test {
     #[case("{:a}")]
     #[case("(not-a-func :uwu)")]
     fn test_evaluator_fail(#[case] input: &str) {
-        let env = &core_environment();
+        let env = core_environment();
         let tokens = lexer::read(input).unwrap();
         let ast = parser::parse(tokens).unwrap();
         let res = eval(env, ast);
diff --git a/src/lexer.rs b/src/lexer.rs
index 34cf0e8..01634cf 100644
--- a/src/lexer.rs
+++ b/src/lexer.rs
@@ -108,7 +108,7 @@ fn next_token(input: &mut Peekable<Chars>) -> Result<Option<Token>> {
             Some(tok) => tok,
             None => return Ok(None),
         },
-        _ => bail!("ilegal token"),
+        _ => bail!("illegal token"),
     };
 
     Ok(Some(tok))
@@ -164,10 +164,11 @@ fn read_int(input: &mut Peekable<Chars>, first: char) -> Token {
 }
 
 fn read_ident(input: &mut Peekable<Chars>, first: char) -> Token {
+    let special_characters = ['!', '?', '*', '-', '_'];
     let mut raw_ident = vec![first];
 
     while let Some(c) = input.peek() {
-        if !c.is_ascii_alphanumeric() {
+        if !c.is_ascii_alphanumeric() && !special_characters.contains(c) {
             break;
         }
 
@@ -198,6 +199,9 @@ mod test {
     #[case("(/ 1 2)", vec![Token::LeftParen, Token::Slash, Token::Int(1), Token::Int(2), Token::RightParen])]
     #[case("(- -2 1)", vec![Token::LeftParen, Token::Minus, Token::Int(-2), Token::Int(1), Token::RightParen])]
     #[case("(\"string and stuff\")", vec![Token::LeftParen, Token::String("string and stuff".into()), Token::RightParen])]
+    #[case("define!", vec![Token::Ident("define!".into())])]
+    #[case("let*", vec![Token::Ident("let*".into())])]
+    #[case("is-int?", vec![Token::Ident("is-int?".into())])]
     #[case(
         "(func a b)",
         vec![
diff --git a/src/main.rs b/src/main.rs
index e45e2df..8ad44a7 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,14 +1,11 @@
 use std::io::{self, Write};
 
-use crate::env::core_environment;
-
-mod env;
-mod eval;
+mod evaluator;
 mod lexer;
 mod parser;
 
 fn main() {
-    let env = core_environment();
+    let env = evaluator::core_environment();
     let mut input = String::new();
 
     println!("MAL -- REPL");
@@ -27,7 +24,7 @@ fn main() {
 
         let tokens = lexer::read(&input).unwrap();
         let ast = parser::parse(tokens).unwrap();
-        let res = eval::eval(&env, ast).unwrap();
+        let res = evaluator::eval(env.clone(), ast).unwrap();
 
         for expr in res {
             println!("{expr}")