diff --git a/dust-lang/src/abstract_tree.rs b/dust-lang/src/abstract_tree.rs
index a734513..dc27996 100644
--- a/dust-lang/src/abstract_tree.rs
+++ b/dust-lang/src/abstract_tree.rs
@@ -37,13 +37,19 @@ pub enum Statement {
     // A sequence of statements
     Block(Vec<Node<Statement>>),
 
-    // Logic, math and comparison expressions
+    // Assignment, logic, math and comparison expressions with two operands
     BinaryOperation {
         left: Box<Node<Statement>>,
         operator: Node<BinaryOperator>,
         right: Box<Node<Statement>>,
     },
 
+    // Logic and math expressions with one operand
+    UnaryOperation {
+        operator: Node<UnaryOperator>,
+        operand: Box<Node<Statement>>,
+    },
+
     // Function calls
     BuiltInFunctionCall {
         function: BuiltInFunction,
@@ -152,6 +158,10 @@ impl Statement {
             }
             Statement::Nil(_) => None,
             Statement::PropertyAccess(_, _) => None,
+            Statement::UnaryOperation { operator, operand } => match operator.inner {
+                UnaryOperator::Negate => Some(operand.inner.expected_type(context)?),
+                UnaryOperator::Not => Some(Type::Boolean),
+            },
             Statement::While { .. } => None,
         }
     }
@@ -331,6 +341,9 @@ impl Display for Statement {
             }
             Statement::Nil(node) => write!(f, "{node};"),
             Statement::PropertyAccess(left, right) => write!(f, "{left}.{right}"),
+            Statement::UnaryOperation { operator, operand } => {
+                write!(f, "{operator}{operand}")
+            }
             Statement::While { condition, body } => {
                 write!(f, "while {condition} {body}")
             }
@@ -383,3 +396,18 @@ impl Display for BinaryOperator {
         }
     }
 }
+
+#[derive(Debug, Clone, Eq, PartialEq, PartialOrd, Ord, Serialize, Deserialize)]
+pub enum UnaryOperator {
+    Negate,
+    Not,
+}
+
+impl Display for UnaryOperator {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        match self {
+            UnaryOperator::Negate => write!(f, "-"),
+            UnaryOperator::Not => write!(f, "!"),
+        }
+    }
+}
diff --git a/dust-lang/src/analyzer.rs b/dust-lang/src/analyzer.rs
index bf529f9..4b2dfe0 100644
--- a/dust-lang/src/analyzer.rs
+++ b/dust-lang/src/analyzer.rs
@@ -11,8 +11,8 @@ use std::{
 };
 
 use crate::{
-    abstract_tree::BinaryOperator, parse, AbstractSyntaxTree, Context, DustError, Node, Span,
-    Statement, Type,
+    abstract_tree::{BinaryOperator, UnaryOperator},
+    parse, AbstractSyntaxTree, Context, DustError, Node, Span, Statement, Type,
 };
 
 /// Analyzes the abstract syntax tree for errors.
@@ -352,6 +352,31 @@ impl<'a> Analyzer<'a> {
                     }
                 }
             }
+            Statement::UnaryOperation { operator, operand } => {
+                self.analyze_statement(operand)?;
+
+                if let UnaryOperator::Negate = operator.inner {
+                    if let Some(Type::Integer | Type::Float | Type::Number) =
+                        operand.inner.expected_type(self.context)
+                    {
+                        // Operand is valid
+                    } else {
+                        return Err(AnalyzerError::ExpectedBoolean {
+                            actual: operand.as_ref().clone(),
+                        });
+                    }
+                }
+
+                if let UnaryOperator::Not = operator.inner {
+                    if let Some(Type::Boolean) = operand.inner.expected_type(self.context) {
+                        // Operand is valid
+                    } else {
+                        return Err(AnalyzerError::ExpectedBoolean {
+                            actual: operand.as_ref().clone(),
+                        });
+                    }
+                }
+            }
             Statement::While { condition, body } => {
                 self.analyze_statement(condition)?;
                 self.analyze_statement(body)?;
diff --git a/dust-lang/src/context.rs b/dust-lang/src/context.rs
index 231cc18..2b34550 100644
--- a/dust-lang/src/context.rs
+++ b/dust-lang/src/context.rs
@@ -133,7 +133,7 @@ mod tests {
     use super::*;
 
     #[test]
-    fn context_removes_used_variables() {
+    fn context_removes_variables() {
         env_logger::builder().is_test(true).try_init().unwrap();
 
         let source = "
@@ -150,7 +150,7 @@ mod tests {
     }
 
     #[test]
-    fn context_removes_variables_after_loop() {
+    fn garbage_collector_does_not_break_loops() {
         let source = "
             y = 1
             z = 0
diff --git a/dust-lang/src/lexer.rs b/dust-lang/src/lexer.rs
index 9dfaa88..421b0ec 100644
--- a/dust-lang/src/lexer.rs
+++ b/dust-lang/src/lexer.rs
@@ -253,6 +253,11 @@ impl Lexer {
                         });
                     }
                 }
+                '!' => {
+                    self.position += 1;
+
+                    (Token::Bang, (self.position - 1, self.position))
+                }
                 _ => {
                     self.position += 1;
 
@@ -479,6 +484,41 @@ impl Display for LexError {
 mod tests {
     use super::*;
 
+    #[test]
+    fn negate_expression() {
+        let input = "x = -42; -x";
+
+        assert_eq!(
+            lex(input),
+            Ok(vec![
+                (Token::Identifier("x"), (0, 1)),
+                (Token::Equal, (2, 3)),
+                (Token::Integer("-42"), (4, 7)),
+                (Token::Semicolon, (7, 8)),
+                (Token::Minus, (9, 10)),
+                (Token::Identifier("x"), (10, 11)),
+                (Token::Eof, (11, 11))
+            ])
+        );
+    }
+
+    #[test]
+    fn not_expression() {
+        let input = "!true; !false";
+
+        assert_eq!(
+            lex(input),
+            Ok(vec![
+                (Token::Bang, (0, 1)),
+                (Token::Boolean("true"), (1, 5)),
+                (Token::Semicolon, (5, 6)),
+                (Token::Bang, (7, 8)),
+                (Token::Boolean("false"), (8, 13)),
+                (Token::Eof, (13, 13))
+            ])
+        );
+    }
+
     #[test]
     fn if_else() {
         let input = "if x < 10 { x + 1 } else { x }";
diff --git a/dust-lang/src/lib.rs b/dust-lang/src/lib.rs
index e84af21..415f3dd 100644
--- a/dust-lang/src/lib.rs
+++ b/dust-lang/src/lib.rs
@@ -14,7 +14,7 @@ pub mod r#type;
 pub mod value;
 pub mod vm;
 
-pub use abstract_tree::{AbstractSyntaxTree, BinaryOperator, Node, Statement};
+pub use abstract_tree::{AbstractSyntaxTree, BinaryOperator, Node, Statement, UnaryOperator};
 pub use analyzer::{analyze, Analyzer, AnalyzerError};
 pub use built_in_function::{BuiltInFunction, BuiltInFunctionError};
 pub use context::{Context, VariableData};
diff --git a/dust-lang/src/parser.rs b/dust-lang/src/parser.rs
index 8e1e8c9..36f9f77 100644
--- a/dust-lang/src/parser.rs
+++ b/dust-lang/src/parser.rs
@@ -13,7 +13,7 @@ use std::{
 
 use crate::{
     AbstractSyntaxTree, BinaryOperator, BuiltInFunction, DustError, Identifier, LexError, Lexer,
-    Node, Span, Statement, Token, TokenOwned, Value,
+    Node, Span, Statement, Token, TokenOwned, UnaryOperator, Value,
 };
 
 /// Parses the input into an abstract syntax tree.
@@ -163,6 +163,34 @@ impl<'src> Parser<'src> {
 
     fn parse_primary(&mut self) -> Result<Node<Statement>, ParseError> {
         match self.current {
+            (Token::Bang, position) => {
+                self.next_token()?;
+
+                let operand = Box::new(self.parse_statement(0)?);
+                let operand_end = operand.position.1;
+
+                Ok(Node::new(
+                    Statement::UnaryOperation {
+                        operator: Node::new(UnaryOperator::Not, position),
+                        operand,
+                    },
+                    (position.0, operand_end),
+                ))
+            }
+            (Token::Minus, position) => {
+                self.next_token()?;
+
+                let operand = Box::new(self.parse_statement(0)?);
+                let operand_end = operand.position.1;
+
+                Ok(Node::new(
+                    Statement::UnaryOperation {
+                        operator: Node::new(UnaryOperator::Negate, position),
+                        operand,
+                    },
+                    (position.0, operand_end),
+                ))
+            }
             (Token::Boolean(text), position) => {
                 self.next_token()?;
 
@@ -799,10 +827,83 @@ impl Display for ParseError {
 
 #[cfg(test)]
 mod tests {
-    use crate::{abstract_tree::BinaryOperator, Identifier};
+    use crate::{abstract_tree::BinaryOperator, Identifier, UnaryOperator};
 
     use super::*;
 
+    #[test]
+    fn negate_variable() {
+        let input = "a = 1; -a";
+
+        assert_eq!(
+            parse(input),
+            Ok(AbstractSyntaxTree {
+                nodes: [
+                    Node::new(
+                        Statement::Nil(Box::new(Node::new(
+                            Statement::BinaryOperation {
+                                left: Box::new(Node::new(
+                                    Statement::Identifier(Identifier::new("a")),
+                                    (0, 1)
+                                )),
+                                operator: Node::new(BinaryOperator::Assign, (2, 3)),
+                                right: Box::new(Node::new(
+                                    Statement::Constant(Value::integer(1)),
+                                    (4, 5)
+                                )),
+                            },
+                            (0, 5)
+                        ))),
+                        (0, 5)
+                    ),
+                    Node::new(
+                        Statement::UnaryOperation {
+                            operator: Node::new(UnaryOperator::Negate, (0, 1)),
+                            operand: Box::new(Node::new(
+                                Statement::Identifier(Identifier::new("a")),
+                                (1, 2)
+                            )),
+                        },
+                        (0, 2)
+                    )
+                ]
+                .into()
+            })
+        );
+    }
+
+    #[test]
+    fn negate_expression() {
+        let input = "-(1 + 1)";
+
+        assert_eq!(
+            parse(input),
+            Ok(AbstractSyntaxTree {
+                nodes: [Node::new(
+                    Statement::UnaryOperation {
+                        operator: Node::new(UnaryOperator::Negate, (0, 1)),
+                        operand: Box::new(Node::new(
+                            Statement::BinaryOperation {
+                                left: Box::new(Node::new(
+                                    Statement::Constant(Value::integer(1)),
+                                    (2, 3)
+                                )),
+                                operator: Node::new(BinaryOperator::Add, (4, 5)),
+                                right: Box::new(Node::new(
+                                    Statement::Constant(Value::integer(1)),
+                                    (6, 7)
+                                )),
+                            },
+                            (1, 8)
+                        )),
+                    },
+                    (0, 8)
+                )]
+                .into()
+            })
+        );
+    }
+
     #[test]
     fn r#if() {
         let input = "if x { y }";
diff --git a/dust-lang/src/token.rs b/dust-lang/src/token.rs
index 09a8e30..882d210 100644
--- a/dust-lang/src/token.rs
+++ b/dust-lang/src/token.rs
@@ -28,6 +28,7 @@ pub enum Token<'src> {
     WriteLine,
 
     // Symbols
+    Bang,
     Comma,
     Dot,
     DoubleAmpersand,
@@ -56,6 +57,7 @@ pub enum Token<'src> {
 impl<'src> Token<'src> {
     pub fn to_owned(&self) -> TokenOwned {
         match self {
+            Token::Bang => TokenOwned::Bang,
             Token::Boolean(boolean) => TokenOwned::Boolean(boolean.to_string()),
             Token::Comma => TokenOwned::Comma,
             Token::Dot => TokenOwned::Dot,
@@ -104,6 +106,7 @@ impl<'src> Token<'src> {
             Token::Integer(integer_text) => integer_text,
             Token::String(text) => text,
 
+            Token::Bang => "!",
             Token::Comma => ",",
             Token::Dot => ".",
             Token::DoubleAmpersand => "&&",
@@ -147,18 +150,19 @@ impl<'src> Token<'src> {
 
     pub fn precedence(&self) -> u8 {
         match self {
-            Token::Dot => 12,
-            Token::Star | Token::Slash | Token::Percent => 10,
-            Token::Plus | Token::Minus => 9,
+            Token::Dot => 9,
+            Token::Star | Token::Slash | Token::Percent => 8,
+            Token::Minus => 7,
+            Token::Plus => 6,
             Token::DoubleEqual
             | Token::Less
             | Token::LessEqual
             | Token::Greater
-            | Token::GreaterEqual => 8,
-            Token::DoubleAmpersand => 7,
-            Token::DoublePipe => 6,
-            Token::Equal | Token::PlusEqual => 5,
-            Token::Semicolon => 4,
+            | Token::GreaterEqual => 5,
+            Token::DoubleAmpersand => 4,
+            Token::DoublePipe => 3,
+            Token::Equal | Token::PlusEqual => 2,
+            Token::Semicolon => 1,
             _ => 0,
         }
     }
@@ -168,7 +172,11 @@ impl<'src> Token<'src> {
     }
 
     pub fn is_right_associative(&self) -> bool {
-        matches!(self, Token::Semicolon)
+        matches!(self, Token::Equal | Token::PlusEqual)
+    }
+
+    pub fn is_prefix(&self) -> bool {
+        matches!(self, Token::Bang | Token::Minus)
     }
 
     pub fn is_postfix(&self) -> bool {
@@ -185,6 +193,7 @@ impl<'src> Display for Token<'src> {
 impl<'src> PartialEq for Token<'src> {
     fn eq(&self, other: &Self) -> bool {
         match (self, other) {
+            (Token::Bang, Token::Bang) => true,
             (Token::Boolean(left), Token::Boolean(right)) => left == right,
             (Token::Comma, Token::Comma) => true,
             (Token::Dot, Token::Dot) => true,
@@ -254,6 +263,7 @@ pub enum TokenOwned {
     WriteLine,
 
     // Symbols
+    Bang,
     Comma,
     Dot,
     DoubleAmpersand,
@@ -282,6 +292,7 @@ pub enum TokenOwned {
 impl Display for TokenOwned {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
         match self {
+            TokenOwned::Bang => Token::Bang.fmt(f),
             TokenOwned::Boolean(boolean) => write!(f, "{boolean}"),
             TokenOwned::Comma => Token::Comma.fmt(f),
             TokenOwned::Dot => Token::Dot.fmt(f),
diff --git a/dust-lang/src/value.rs b/dust-lang/src/value.rs
index 15ba230..07b3c5d 100644
--- a/dust-lang/src/value.rs
+++ b/dust-lang/src/value.rs
@@ -105,6 +105,14 @@ impl Value {
         }
     }
 
+    pub fn as_float(&self) -> Option<f64> {
+        if let ValueInner::Float(float) = self.0.as_ref() {
+            Some(*float)
+        } else {
+            None
+        }
+    }
+
     pub fn as_function(&self) -> Option<&Function> {
         if let ValueInner::Function(function) = self.0.as_ref() {
             Some(function)
diff --git a/dust-lang/src/vm.rs b/dust-lang/src/vm.rs
index 768d3bd..b4075b3 100644
--- a/dust-lang/src/vm.rs
+++ b/dust-lang/src/vm.rs
@@ -6,8 +6,8 @@ use std::{
 
 use crate::{
     abstract_tree::BinaryOperator, parse, value::ValueInner, AbstractSyntaxTree, Analyzer,
-    BuiltInFunctionError, Context, DustError, Identifier, Node, ParseError, Span, Statement, Value,
-    ValueError,
+    BuiltInFunctionError, Context, DustError, Identifier, Node, ParseError, Span, Statement,
+    UnaryOperator, Value, ValueError,
 };
 
 pub fn run(source: &str) -> Result<Option<Value>, DustError> {
@@ -500,6 +500,33 @@ impl Vm {
                     position: right_span,
                 })
             }
+            Statement::UnaryOperation { operator, operand } => {
+                let position = operand.position;
+                let value = if let Some(value) = self.run_statement(*operand, context)? {
+                    value
+                } else {
+                    return Err(VmError::ExpectedValue { position });
+                };
+
+                match operator.inner {
+                    UnaryOperator::Negate => {
+                        if let Some(value) = value.as_integer() {
+                            Ok(Some(Value::integer(-value)))
+                        } else if let Some(value) = value.as_float() {
+                            Ok(Some(Value::float(-value)))
+                        } else {
+                            Err(VmError::ExpectedNumber { position })
+                        }
+                    }
+                    UnaryOperator::Not => {
+                        if let Some(value) = value.as_boolean() {
+                            Ok(Some(Value::boolean(!value)))
+                        } else {
+                            Err(VmError::ExpectedBoolean { position })
+                        }
+                    }
+                }
+            }
             Statement::While { condition, body } => {
                 let mut return_value = None;
 
@@ -555,6 +582,9 @@ pub enum VmError {
     ExpectedInteger {
         position: Span,
     },
+    ExpectedNumber {
+        position: Span,
+    },
     ExpectedFunction {
         actual: Value,
         position: Span,
@@ -588,6 +618,7 @@ impl VmError {
             Self::ExpectedInteger { position } => *position,
             Self::ExpectedFunction { position, .. } => *position,
             Self::ExpectedList { position } => *position,
+            Self::ExpectedNumber { position } => *position,
             Self::ExpectedValue { position } => *position,
             Self::UndefinedVariable { identifier } => identifier.position,
             Self::UndefinedProperty {
@@ -637,6 +668,13 @@ impl Display for VmError {
             Self::ExpectedList { position } => {
                 write!(f, "Expected a list at position: {:?}", position)
             }
+            Self::ExpectedNumber { position } => {
+                write!(
+                    f,
+                    "Expected an integer or float at position: {:?}",
+                    position
+                )
+            }
             Self::ExpectedValue { position } => {
                 write!(f, "Expected a value at position: {:?}", position)
             }
@@ -656,6 +694,20 @@ impl Display for VmError {
 mod tests {
     use super::*;
 
+    #[test]
+    fn negate_expression() {
+        let input = "x = -42; -x";
+
+        assert_eq!(run(input), Ok(Some(Value::integer(42))));
+    }
+
+    #[test]
+    fn not_expression() {
+        let input = "!(1 == 2 || 3 == 4 || 5 == 6)";
+
+        assert_eq!(run(input), Ok(Some(Value::boolean(true))));
+    }
+
     #[test]
     fn list_index() {
         let input = "[1, 42, 3].1";