diff --git a/src/frontend/ast.rs b/src/frontend/ast.rs
index 27e843f..6ab95fc 100644
--- a/src/frontend/ast.rs
+++ b/src/frontend/ast.rs
@@ -5,7 +5,6 @@ pub trait Expr: Stmt {}
 pub enum NodeType {
     Program(Program),
     NumericLiteral(NumericLiteral),
-    NullLiteral(NullLiteral),
     Identifier(Identifier),
     BinaryExpr(BinaryExpr),
     EOF,
@@ -33,9 +32,6 @@ pub struct NumericLiteral {
     pub value: f64,
 }
 
-#[derive(Debug)]
-pub struct NullLiteral {}
-
 impl Stmt for Program {}
 
 impl Stmt for NumericLiteral {}
diff --git a/src/frontend/lexer.rs b/src/frontend/lexer.rs
index 505ec39..f3f4f73 100644
--- a/src/frontend/lexer.rs
+++ b/src/frontend/lexer.rs
@@ -27,14 +27,12 @@ pub mod interpreter {
         CloseParen,
         OpenParen,
         Equals,
-        Null,
         Let,
         EOF,
     }
 
     static KEYWORDS: phf::Map<&'static str, Token> = phf::phf_map! {
         "let" => Token::Let,
-        "null" => Token::Null,
     };
 
     impl Token {
@@ -81,7 +79,6 @@ pub mod interpreter {
                 Token::OpenParen => write!(f, "Token::OpenParen"),
                 Token::CloseParen => write!(f, "Token::CloseParen"),
                 Token::EOF => write!(f, "Token::EOF"),
-                Token::Null => write!(f, "Token::Null"),
             }
         }
     }
diff --git a/src/frontend/parser.rs b/src/frontend/parser.rs
index 04f9ca0..c420047 100644
--- a/src/frontend/parser.rs
+++ b/src/frontend/parser.rs
@@ -1,6 +1,6 @@
 use std::process;
 
-use crate::ast::{BinaryExpr, Identifier, NodeType, NullLiteral, NumericLiteral, Program};
+use crate::ast::{BinaryExpr, Identifier, NodeType, NumericLiteral, Program};
 use crate::lexer::interpreter;
 
 pub struct Parser {
@@ -50,7 +50,6 @@ impl Parser {
             Token::Number(n) => NodeType::NumericLiteral(NumericLiteral {
                 value: n.parse().expect(&format!("Failed to parse number: {}", n)),
             }),
-            Token::Null => NodeType::NullLiteral(NullLiteral {}),
             Token::EOF => NodeType::EOF,
 
             Token::OpenParen => {
diff --git a/src/main.rs b/src/main.rs
index 61cf11f..acad012 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,4 +1,5 @@
 use clap::Parser;
+use std::io::Write;
 use std::process;
 
 mod frontend;
@@ -7,11 +8,32 @@ use frontend::lexer;
 use frontend::parser;
 
 mod runtime;
+use runtime::environment::Environment;
 use runtime::interpreter::evaluate;
 
+macro_rules! declare_var {
+    ($env:ident, $name:expr, $value:expr) => {
+        $env.declare_var(String::from($name), $value)
+            .expect(&format!(
+                "Failed to declare variable '{}' in the REPL environment.",
+                $name
+            ));
+    };
+}
+
 pub fn repl() {
     // Read-Eval-Print Loop
-    use std::io::{self, Write};
+    use std::{cell::RefCell, io, rc::Rc};
+
+    let mut env = Environment::new();
+
+    declare_var!(env, "x", make_num!(10.0));
+
+    declare_var!(env, "true", make_bool!(true));
+    declare_var!(env, "false", make_bool!(false));
+    declare_var!(env, "null", make_null!());
+
+    let env = Rc::new(RefCell::new(env));
 
     println!("Welcome to the Riv Lang REPL! Type 'exit' to quit.");
 
@@ -30,7 +52,7 @@ pub fn repl() {
 
         let program = parser::Parser::new(input).produce_ast();
 
-        let result = evaluate(frontend::ast::NodeType::Program(program));
+        let result = evaluate(frontend::ast::NodeType::Program(program), &env);
         println!("{:#?}", result);
     }
 }
diff --git a/src/runtime/environment.rs b/src/runtime/environment.rs
new file mode 100644
index 0000000..389607b
--- /dev/null
+++ b/src/runtime/environment.rs
@@ -0,0 +1,87 @@
+use crate::runtime::values::ValueType;
+use std::cell::RefCell;
+use std::collections::HashMap;
+use std::rc::Rc;
+
+#[derive(Debug, Clone)]
+pub struct Environment {
+    parent: Option<Rc<RefCell<Environment>>>,
+    variables: HashMap<String, ValueType>,
+}
+
+impl Environment {
+    /// Creates a new root environment with no parent.
+    pub fn new() -> Self {
+        Self {
+            parent: None,
+            variables: HashMap::new(),
+        }
+    }
+
+    /// Creates a new child environment wrapped in Rc<RefCell<...>>
+    /// so it can be passed down safely to deeper scopes.
+    pub fn new_child(parent: Rc<RefCell<Environment>>) -> Rc<RefCell<Self>> {
+        Rc::new(RefCell::new(Self {
+            parent: Some(parent),
+            variables: HashMap::new(),
+        }))
+    }
+
+    pub fn declare_var(&mut self, varname: String, value: ValueType) -> Result<ValueType, String> {
+        if self.variables.contains_key(&varname) {
+            return Err(format!(
+                "Cannot declare variable '{}'. As it already is defined.",
+                varname
+            ));
+        }
+
+        self.variables.insert(varname, value.clone());
+        Ok(value)
+    }
+
+    pub fn assign_var(
+        env_ref: &Rc<RefCell<Self>>,
+        varname: String,
+        value: ValueType,
+    ) -> Result<ValueType, String> {
+        let resolved_env = Self::resolve(env_ref, &varname)?;
+        resolved_env
+            .borrow_mut()
+            .variables
+            .insert(varname, value.clone());
+        Ok(value)
+    }
+
+    pub fn lookup_var(env_ref: &Rc<RefCell<Self>>, varname: &str) -> Result<ValueType, String> {
+        let resolved_env = Self::resolve(env_ref, varname)?;
+        // We clone the value out to avoid holding a reference borrow
+        let val = resolved_env
+            .borrow()
+            .variables
+            .get(varname)
+            .cloned()
+            .unwrap();
+        Ok(val)
+    }
+
+    pub fn resolve(
+        env_ref: &Rc<RefCell<Self>>,
+        varname: &str,
+    ) -> Result<Rc<RefCell<Self>>, String> {
+        if env_ref.borrow().variables.contains_key(varname) {
+            return Ok(Rc::clone(env_ref));
+        }
+
+        // Access the parent if it exists
+        let parent_opt = env_ref.borrow().parent.as_ref().map(Rc::clone);
+
+        if let Some(parent_env) = parent_opt {
+            Self::resolve(&parent_env, varname)
+        } else {
+            Err(format!(
+                "Cannot resolve '{}' as it does not exist.",
+                varname
+            ))
+        }
+    }
+}
diff --git a/src/runtime/interpreter.rs b/src/runtime/interpreter.rs
index f07cd73..6b99165 100644
--- a/src/runtime/interpreter.rs
+++ b/src/runtime/interpreter.rs
@@ -1,27 +1,31 @@
+use std::cell::RefCell;
 use std::process;
+use std::rc::Rc;
 
 use crate::frontend::ast::{BinaryExpr, NodeType, Program};
+use crate::runtime::environment::Environment;
 use crate::runtime::values::ValueType;
+use crate::{make_null, make_num};
 
-fn eval_program(program: Program) -> ValueType {
+fn eval_program(program: Program, env: &Rc<RefCell<Environment>>) -> ValueType {
     // Initialize with your equivalent of NullVal
-    let mut last_evaluated = ValueType::Null;
+    let mut last_evaluated = make_null!();
 
     // Iterate through each statement in the program body
     for statement in program.body {
-        last_evaluated = evaluate(statement);
+        last_evaluated = evaluate(statement, env);
     }
 
     last_evaluated
 }
 
-fn eval_binop(binop: BinaryExpr) -> ValueType {
-    let lhs = evaluate(*binop.left);
-    let rhs = evaluate(*binop.right);
+fn eval_binop(binop: BinaryExpr, env: &Rc<RefCell<Environment>>) -> ValueType {
+    let lhs = evaluate(*binop.left, env);
+    let rhs = evaluate(*binop.right, env);
 
     match (&lhs, &rhs) {
         (ValueType::Number(l), ValueType::Number(r)) => eval_num_binop(*l, *r, binop.operator),
-        _ => ValueType::Null,
+        _ => make_null!(),
     }
 }
 
@@ -41,15 +45,28 @@ fn eval_num_binop(lhs: f64, rhs: f64, operator: String) -> ValueType {
         }
     }
 
-    ValueType::Number(result)
+    make_num!(result)
 }
 
-pub fn evaluate(node: NodeType) -> ValueType {
+// Change `env: &Rc<RefCell<Environment>>` or `env: &Environment`
+// to `env: &Rc<RefCell<Environment>>`
+fn eval_ident(ident: String, env: &Rc<RefCell<Environment>>) -> ValueType {
+    // Now &env perfectly matches the expected `&Rc<RefCell<Environment>>`
+    match Environment::lookup_var(env, &ident) {
+        Ok(value) => value,
+        Err(err) => {
+            eprintln!("Error looking up identifier '{}': {}", ident, err);
+            std::process::exit(1);
+        }
+    }
+}
+
+pub fn evaluate(node: NodeType, env: &Rc<RefCell<Environment>>) -> ValueType {
     match node {
-        NodeType::NumericLiteral(n) => ValueType::Number(n.value),
-        NodeType::NullLiteral(_) => ValueType::Null,
-        NodeType::BinaryExpr(e) => eval_binop(e),
-        NodeType::Program(p) => eval_program(p),
+        NodeType::NumericLiteral(n) => make_num!(n.value),
+        NodeType::BinaryExpr(e) => eval_binop(e, env),
+        NodeType::Program(p) => eval_program(p, env),
+        NodeType::Identifier(i) => eval_ident(i.symbol, env),
         _ => {
             eprintln!(
                 "Evaluation for node type {:#?} is not implemented yet.",
diff --git a/src/runtime/mod.rs b/src/runtime/mod.rs
index 27bd8a9..1ae9de1 100644
--- a/src/runtime/mod.rs
+++ b/src/runtime/mod.rs
@@ -1,2 +1,3 @@
+pub mod environment;
 pub mod interpreter;
 pub mod values;
diff --git a/src/runtime/values.rs b/src/runtime/values.rs
index b4f5be7..c11b342 100644
--- a/src/runtime/values.rs
+++ b/src/runtime/values.rs
@@ -2,4 +2,26 @@
 pub enum ValueType {
     Null,
     Number(f64),
+    Boolean(bool),
+}
+
+#[macro_export]
+macro_rules! make_num {
+    ($n: expr) => {
+        crate::runtime::values::ValueType::Number($n)
+    };
+}
+
+#[macro_export]
+macro_rules! make_null {
+    () => {
+        crate::runtime::values::ValueType::Null
+    };
+}
+
+#[macro_export]
+macro_rules! make_bool {
+    ($b: expr) => {
+        crate::runtime::values::ValueType::Boolean($b)
+    };
 }