diff --git a/src/entry/unix/unix.rs b/src/entry/unix/unix.rs
index cfe2c89..d101a59 100644
--- a/src/entry/unix/unix.rs
+++ b/src/entry/unix/unix.rs
@@ -24,7 +24,7 @@ use crate::context::Context;
 fn do_expression(
 	stdout: &mut RawTerminal<std::io::Stdout>,
 	s: &String,
-	context: &Context
+	context: &mut Context
 ) -> Result<parser::Token, ()> {
 	#[cfg(debug_assertions)]
 	RawTerminal::suspend_raw_mode(&stdout).unwrap();
@@ -118,6 +118,16 @@ fn do_expression(
 					style::Reset,
 					color::Fg(color::Reset),
 				).unwrap();
+			},
+
+			Err(EvalError::BadDefineName) => {
+				write!(
+					stdout, "\n  {}{}Evaluation Error: {}Invalid variable name{}\r\n\n",
+					style::Bold,
+					color::Fg(color::Red),
+					style::Reset,
+					color::Fg(color::Reset),
+				).unwrap();
 			}
 		}
 	}
@@ -167,7 +177,7 @@ pub fn main() -> Result<(), std::io::Error> {
 						} else if command::is_command(&in_str) {
 							command::do_command(&mut stdout, &in_str)?;
 						} else {
-							let r = do_expression(&mut stdout, &in_str, &context);
+							let r = do_expression(&mut stdout, &in_str, &mut context);
 							if let Ok(t) = r { context.push_hist(t); }
 						}
 
diff --git a/src/evaluate/evaluate.rs b/src/evaluate/evaluate.rs
index 85cacc6..dfa027f 100644
--- a/src/evaluate/evaluate.rs
+++ b/src/evaluate/evaluate.rs
@@ -26,7 +26,9 @@ pub fn evaluate(t: &Token, context: &mut Context) -> Result<Token, EvalError> {
 	// Exits when we finish parsing the root node.
 	loop {
 		// Current position in the tree
-		let g = root.get_at_coords_mut(&coords[0 .. coords.len() - 1]);
+		let g = root.get_at_coords_mut(
+			&coords[0 .. coords.len() - 1]
+		).unwrap();
 
 		// "Move up" step.
 		// We move up if we're at a leaf or if we're out of children to move down to.
@@ -36,16 +38,18 @@ pub fn evaluate(t: &Token, context: &mut Context) -> Result<Token, EvalError> {
 			(coords.len() != 0 && (*coords.last().unwrap() >= g.get_args().unwrap().len()))
 		} {
 
-			if !g.is_quantity() {
-				*g = match g {
-					Token::Quantity(_) => panic!(),
 
-					Token::Constant(c) => { evaluate(&c.value(), context).unwrap() },
-					Token::Variable(s) => { context.get_variable(&s).unwrap() },
-					Token::Operator(Operator::Function(f), v) => { eval_function(&f, &v)? },
-					Token::Operator(o, v) => { eval_operator(&o, &v, context)? },
-				}
-			}
+			let new = match g {
+				Token::Quantity(_) => None,
+
+				Token::Constant(c) => { Some(evaluate(&c.value(), context).unwrap()) },
+				Token::Variable(s) => { context.get_variable(&s) },
+				Token::Operator(Operator::Function(f), v) => { Some(eval_function(&f, &v)?) },
+				Token::Operator(o, v) => { eval_operator(&o, &v, context)? },
+			};
+
+			if new.is_some() { *g = new.unwrap()}
+
 
 			// Move up the tree
 			coords.pop();
@@ -56,6 +60,13 @@ pub fn evaluate(t: &Token, context: &mut Context) -> Result<Token, EvalError> {
 		} else {
 			// Move down the tree
 			coords.push(0);
+
+			let n = root.get_at_coords(&coords[..]);
+			if let Some(n) = n {
+				if let Token::Operator(Operator::Define, _) = n {
+					*coords.last_mut().unwrap() += 1;
+				}
+			}
 		}
 	}
 
diff --git a/src/evaluate/mod.rs b/src/evaluate/mod.rs
index 4e5cca9..fe5c65b 100644
--- a/src/evaluate/mod.rs
+++ b/src/evaluate/mod.rs
@@ -9,5 +9,6 @@ pub enum EvalError {
 	BadMath,
 	TooBig,
 	ZeroDivision,
-	IncompatibleUnit
+	IncompatibleUnit,
+	BadDefineName
 }
\ No newline at end of file
diff --git a/src/evaluate/operator.rs b/src/evaluate/operator.rs
index c4e7959..3907c99 100644
--- a/src/evaluate/operator.rs
+++ b/src/evaluate/operator.rs
@@ -4,8 +4,9 @@ use crate::quantity::Quantity;
 use crate::parser::Operator;
 use crate::parser::Token;
 use super::EvalError;
+use crate::context::Context;
 
-pub fn eval_operator(op: &Operator, args: &VecDeque<Token>) -> Result<Token, EvalError> {
+pub fn eval_operator(op: &Operator, args: &VecDeque<Token>, context: &mut Context) -> Result<Option<Token>, EvalError> {
 	match op {
 
 		// Handled seperately in evaluate.rs
@@ -19,32 +20,42 @@ pub fn eval_operator(op: &Operator, args: &VecDeque<Token>) -> Result<Token, Eva
 		Operator::DivideLong |
 		Operator::Subtract => { panic!() }
 
+		Operator::Define => {
+			if args.len() != 2 { panic!() };
+			let b = &args[1];
+
+			if let Token::Variable(s) = &args[0] {
+				context.push_var(s.clone(), b.clone());
+				return Ok(Some(b.clone()));
+			} else { return Err(EvalError::BadDefineName); }
+		},
+
 		Operator::Negative => {
-			if args.len() != 1 {panic!()};
+			if args.len() != 1 { panic!() };
 			let args = &args[0];
 
 			if let Token::Quantity(v) = args {
-				return Ok(Token::Quantity(-v.clone()));
-			} else { panic!(); }
+				return Ok(Some(Token::Quantity(-v.clone())));
+			} else { return Ok(None); }
 		},
 
 		Operator::Flip => {
-			if args.len() != 1 {panic!()};
+			if args.len() != 1 { panic!() };
 			let args = &args[0];
 
 			if let Token::Quantity(v) = args {
 				if v.is_zero() { return Err(EvalError::ZeroDivision); }
-				return Ok(Token::Quantity(
+				return Ok(Some(Token::Quantity(
 					Quantity::new_rational(1f64).unwrap()/v.clone()
-				));
-			} else { panic!(); }
+				)));
+			} else { return Ok(None); }
 		},
 
 		Operator::Add => {
 			let mut sum: Quantity;
 			if let Token::Quantity(s) = &args[0] {
 				sum = s.clone();
-			} else {panic!()};
+			} else { return Ok(None); };
 
 			let mut i: usize = 1;
 			while i < args.len() {
@@ -56,12 +67,10 @@ pub fn eval_operator(op: &Operator, args: &VecDeque<Token>) -> Result<Token, Eva
 					}
 
 					sum += v.clone();
-				} else {
-					panic!();
-				}
+				} else { return Ok(None); }
 				i += 1;
 			}
-			return Ok(Token::Quantity(sum));
+			return Ok(Some(Token::Quantity(sum)));
 		},
 
 		Operator::Multiply => {
@@ -70,16 +79,14 @@ pub fn eval_operator(op: &Operator, args: &VecDeque<Token>) -> Result<Token, Eva
 				let j = i;
 				if let Token::Quantity(v) = j {
 					prod *= v.clone();
-				} else {
-					panic!();
-				}
+				} else { return Ok(None); }
 			}
-			return Ok(Token::Quantity(prod));
+			return Ok(Some(Token::Quantity(prod)));
 		},
 
 		Operator::ModuloLong
 		| Operator::Modulo => {
-			if args.len() != 2 {panic!()};
+			if args.len() != 2 { panic!() };
 			let a = &args[0];
 			let b = &args[1];
 
@@ -94,14 +101,14 @@ pub fn eval_operator(op: &Operator, args: &VecDeque<Token>) -> Result<Token, Eva
 					if va.fract() != Quantity::new_rational(0f64).unwrap() { return Err(EvalError::BadMath); }
 					if vb.fract() != Quantity::new_rational(0f64).unwrap() { return Err(EvalError::BadMath); }
 
-					return Ok(Token::Quantity(va.clone() % vb.clone()));
-				} else { panic!(); }
-			} else { panic!(); }
+					return Ok(Some(Token::Quantity(va.clone() % vb.clone())));
+				} else { return Ok(None); }
+			} else { return Ok(None); }
 		},
 
 		Operator::UnitConvert
 		=> {
-			if args.len() != 2 {panic!()};
+			if args.len() != 2 { panic!() };
 			let a = &args[0];
 			let b = &args[1];
 
@@ -111,9 +118,9 @@ pub fn eval_operator(op: &Operator, args: &VecDeque<Token>) -> Result<Token, Eva
 					if n.is_none() {
 						return Err(EvalError::IncompatibleUnit);
 					}
-					return Ok(Token::Quantity(n.unwrap()));
-				} else { panic!(); }
-			} else { panic!(); }
+					return Ok(Some(Token::Quantity(n.unwrap())));
+				} else { return Ok(None); }
+			} else { return Ok(None); }
 		},
 
 		Operator::Power => {
@@ -134,9 +141,9 @@ pub fn eval_operator(op: &Operator, args: &VecDeque<Token>) -> Result<Token, Eva
 
 					let p = va.pow(vb.clone());
 					if p.is_nan() {return Err(EvalError::BadMath);}
-					return Ok(Token::Quantity(p));
-				} else { panic!(); }
-			} else { panic!(); }
+					return Ok(Some(Token::Quantity(p)));
+				} else { return Ok(None); }
+			} else { return Ok(None); }
 		},
 
 		Operator::Factorial => {
@@ -159,8 +166,8 @@ pub fn eval_operator(op: &Operator, args: &VecDeque<Token>) -> Result<Token, Eva
 					u = u - Quantity::new_rational(1f64).unwrap();
 				}
 
-				return Ok(Token::Quantity(prod));
-			} else { panic!(); }
+				return Ok(Some(Token::Quantity(prod)));
+			} else { return Ok(None); }
 		}
 	};
 }
\ No newline at end of file
diff --git a/src/parser/parsererror.rs b/src/parser/parsererror.rs
index b6c6701..3666ae4 100644
--- a/src/parser/parsererror.rs
+++ b/src/parser/parsererror.rs
@@ -15,7 +15,6 @@ pub enum ParserError {
 	ExtraCloseParen,
 	EmptyGroup,
 	Syntax,
-	Undefined(String),
 	BadNumber
 }
 
@@ -34,9 +33,6 @@ impl ToString for ParserError {
 			ParserError::Syntax => {
 				String::from("Syntax")
 			},
-			ParserError::Undefined(s) => {
-				format!("\"{s}\" isn't defined")
-			},
 			ParserError::BadNumber => {
 				String::from("Invalid number")
 			}
diff --git a/src/parser/pretoken.rs b/src/parser/pretoken.rs
index f151467..15bd070 100644
--- a/src/parser/pretoken.rs
+++ b/src/parser/pretoken.rs
@@ -68,13 +68,15 @@ impl PreToken {
 				}
 
 				let r = Quantity::new_rational_from_string(&s);
+
 				if r.is_none() {
 					return Err((l, ParserError::BadNumber))
 				}
+
 				return Ok(Token::Quantity(r.unwrap()));
 			},
 
-			PreToken::PreWord(l, s) => {
+			PreToken::PreWord(_l, s) => {
 
 				let c = Constant::from_string(&s);
 				if c.is_some() { return Ok(Token::Constant(c.unwrap())); }
@@ -84,8 +86,7 @@ impl PreToken {
 
 				let c = context.get_variable(&s);
 				if c.is_some() { return Ok(Token::Variable(s)); }
-
-				return Err((l, ParserError::Undefined(s)));
+				return Ok(Token::Variable(s));
 			}
 
 			PreToken::Container(v) => { return Ok(v); }
diff --git a/src/parser/stage/tokenize.rs b/src/parser/stage/tokenize.rs
index 5d705d7..231421b 100644
--- a/src/parser/stage/tokenize.rs
+++ b/src/parser/stage/tokenize.rs
@@ -136,7 +136,7 @@ pub fn tokenize(input: &String) -> VecDeque<PreToken> {
 
 			// Operator
 			'*'|'×'|'/'|'÷'|
-			'^'|'!'|'%'
+			'^'|'!'|'%'|'='
 			=> {
 				match &mut t {
 					Some(PreToken::PreOperator(_, val)) => { val.push(c); },
diff --git a/src/parser/token/operator.rs b/src/parser/token/operator.rs
index 7cd0bd2..08bac28 100644
--- a/src/parser/token/operator.rs
+++ b/src/parser/token/operator.rs
@@ -11,7 +11,8 @@ use super::Function;
 #[derive(Clone)]
 #[repr(usize)]
 pub enum Operator {
-	ModuloLong = 0, // Mod invoked with "mod"
+	Define = 0, // Variable and function definition
+	ModuloLong, // Mod invoked with "mod"
 	DivideLong,
 	UnitConvert,
 	Subtract,
@@ -68,6 +69,7 @@ impl Operator {
 		}
 
 		return match s {
+			"="      => {Some( Operator::Define )},
 			"+"      => {Some( Operator::Add )},
 			"-"      => {Some( Operator::Subtract )},
 			"neg"    => {Some( Operator::Negative )},
@@ -166,6 +168,7 @@ impl Operator {
 			| Operator::Power
 			| Operator::ModuloLong
 			| Operator::UnitConvert
+			| Operator::Define
 			=> { Token::Operator(self, args) },
 		}
 	}
@@ -201,6 +204,14 @@ impl Operator {
 			Operator::Divide |
 			Operator::Subtract => { panic!() }
 
+			Operator::Define => {
+				return format!(
+					"{} = {}",
+					self.add_parens_to_arg(&args[0]),
+					self.add_parens_to_arg(&args[1])
+				);
+			},
+
 			Operator::Flip => {
 				return format!("{}⁻¹", Operator::Divide.add_parens_to_arg(&args[0]));
 			},
diff --git a/src/parser/token/token.rs b/src/parser/token/token.rs
index eb3f0ef..386156d 100644
--- a/src/parser/token/token.rs
+++ b/src/parser/token/token.rs
@@ -37,6 +37,13 @@ impl Token {
 		}
 	}
 
+	pub fn is_quantity(&self) -> bool {
+		match self {
+			Token::Quantity(_) => true,
+			_ => false
+		}
+	}
+
 	#[inline(always)]
 	pub fn get_args_mut(&mut self) -> Option<&mut VecDeque<Token>> {
 		match self {
@@ -46,14 +53,34 @@ impl Token {
 	}
 
 	#[inline(always)]
-	pub fn get_at_coords<'a>(g: &'a mut Token, coords: &Vec<usize>) -> &'a mut Token {
-		let mut h = &mut *g;
-
-		for t in coords.iter() {
-			let inner = h.get_args_mut().unwrap();
-			h = &mut inner[*t];
+	pub fn get_args(&self) -> Option<&VecDeque<Token>> {
+		match self {
+			Token::Operator(_, ref a) => Some(a),
+			_ => None
 		}
+	}
 
-		return h;
+	#[inline(always)]
+	pub fn get_at_coords<'a, 'b, I>(&'a self, coords: I) -> Option<&'a Token>
+	where I: IntoIterator<Item = &'b usize> + Sized {
+		let mut g = self;
+		for t in coords.into_iter() {
+			let args = g.get_args();
+			let Some(args) = args else { return None; };
+			g = &args[*t];
+		}
+		return Some(g);
+	}
+
+	#[inline(always)]
+	pub fn get_at_coords_mut<'a, 'b, I>(&'a mut self, coords: I) -> Option<&'a mut Token>
+	where I: IntoIterator<Item = &'b usize> + Sized {
+		let mut g = self;
+		for t in coords.into_iter() {
+			let args = g.get_args_mut();
+			let Some(args) = args else { return None; };
+			g = &mut args[*t];
+		}
+		return Some(g);
 	}
 }
\ No newline at end of file
diff --git a/src/tests.rs b/src/tests.rs
index b677739..b245173 100644
--- a/src/tests.rs
+++ b/src/tests.rs
@@ -10,7 +10,7 @@ fn eval_to_str(s: &str) -> Result<String, ()> {
 	};
 	//let out_str = g.print();
 
-	return match evaluate(&g, &Context::new()) {
+	return match evaluate(&g, &mut Context::new()) {
 		Ok(x) => Ok(x.to_string_outer()),
 		Err(_) => Err(())
 	};