diff --git a/src/expr.zig b/src/expr.zig
index 11fecec..b13c1a1 100644
--- a/src/expr.zig
+++ b/src/expr.zig
@@ -24,10 +24,11 @@ pub const LiteralTag = enum {
 };
 
 pub const LiteralExpr = union(LiteralTag) {
-    number: u64,
-    string: []u8,
+    number: f64,
+    string: []const u8,
     boolean: bool,
-    nil: void,
+    // FIXME: See if there is a way to make this void.
+    nil: bool,
 };
 
 pub const UnaryExpr = struct {
diff --git a/src/main.zig b/src/main.zig
index 5492ee6..22a7a2c 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -1,6 +1,8 @@
 const std = @import("std");
 
 const scanner = @import("scanner.zig");
+const expr = @import("expr.zig");
+const parser = @import("parser.zig");
 const err = @import("error.zig");
 
 pub fn main() !void {
@@ -56,8 +58,17 @@ fn runPrompt(alloc: std.mem.Allocator) !void {
 fn run(allocator: std.mem.Allocator, bytes: []u8) !void {
     var scan = scanner.Scanner.init(allocator, bytes);
     defer scan.deinit();
-    std.debug.print("{any}\n", .{scan.scanTokens()});
-}
 
-// Error reporting
-// TODO: Move to a separate file.
+    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
+    defer arena.deinit();
+    var alloc = arena.allocator();
+    var parse = parser.Parser{
+        .tokens = scan.scanTokens(),
+        .allocator = alloc,
+    };
+
+    const expression = try parse.expression();
+    // std.debug.print("AST: {}", .{expression.*});
+    expr.AstPrint(expression.*);
+    std.debug.print("\n", .{});
+}
diff --git a/src/parser.zig b/src/parser.zig
new file mode 100644
index 0000000..849baa2
--- /dev/null
+++ b/src/parser.zig
@@ -0,0 +1,181 @@
+const std = @import("std");
+
+const expr_zig = @import("expr.zig");
+const Expr = expr_zig.Expr;
+const BinaryExpr = expr_zig.BinaryExpr;
+const UnaryExpr = expr_zig.UnaryExpr;
+const LiteralExpr = expr_zig.LiteralExpr;
+const GroupingExpr = expr_zig.GroupingExpr;
+
+const token_zig = @import("token.zig");
+const TokenType = token_zig.TokenType;
+const Token = token_zig.Token;
+
+const errors_zig = @import("error.zig");
+const err = errors_zig.err;
+
+pub const Parser = struct {
+    const Self = @This();
+    tokens: std.ArrayList(Token),
+    allocator: std.mem.Allocator,
+    current: u64 = 0,
+
+    pub fn expression(self: *Self) !*Expr {
+        return self.equality();
+    }
+
+    pub fn equality(self: *Self) !*Expr {
+        var expr = try self.comparison();
+
+        while (true) {
+            switch (self.peekType()) {
+                TokenType.BANG_EQUAL, TokenType.EQUAL_EQUAL => {
+                    var old_expr = expr;
+                    expr = try self.allocator.create(Expr);
+                    expr.* = Expr{
+                        .binary = BinaryExpr{
+                            .operator = self.advance(),
+                            .left = old_expr,
+                            .right = try self.comparison(),
+                        },
+                    };
+                },
+                else => break,
+            }
+        }
+        return expr;
+    }
+
+    fn comparison(self: *Self) !*Expr {
+        var expr = try self.term();
+
+        while (true) {
+            switch (self.peekType()) {
+                TokenType.GREATER, TokenType.GREATER_EQUAL, TokenType.LESS, TokenType.LESS_EQUAL => {
+                    var old_expr = expr;
+                    expr = try self.allocator.create(Expr);
+                    expr.* = Expr{
+                        .binary = BinaryExpr{
+                            .operator = self.advance(),
+                            .left = old_expr,
+                            .right = try self.term(),
+                        },
+                    };
+                },
+                else => break,
+            }
+        }
+        return expr;
+    }
+
+    fn term(self: *Self) !*Expr {
+        var expr = try self.factor();
+
+        while (true) {
+            switch (self.peekType()) {
+                TokenType.PLUS, TokenType.MINUS => {
+                    var old_expr = expr;
+                    expr = try self.allocator.create(Expr);
+                    expr.* = Expr{
+                        .binary = BinaryExpr{
+                            .operator = self.advance(),
+                            .left = old_expr,
+                            .right = try self.factor(),
+                        },
+                    };
+                },
+                else => break,
+            }
+        }
+        return expr;
+    }
+
+    fn factor(self: *Self) !*Expr {
+        var expr = try self.unary();
+
+        while (true) {
+            switch (self.peekType()) {
+                TokenType.STAR, TokenType.SLASH => {
+                    var old_expr = expr;
+                    expr = try self.allocator.create(Expr);
+                    expr.* = Expr{
+                        .binary = BinaryExpr{
+                            .operator = self.advance(),
+                            .left = old_expr,
+                            .right = try self.unary(),
+                        },
+                    };
+                },
+                else => break,
+            }
+        }
+        return expr;
+    }
+
+    fn unary(self: *Self) !*Expr {
+        switch (self.peekType()) {
+            TokenType.BANG, TokenType.MINUS => {
+                var expr = try self.allocator.create(Expr);
+                expr.* = Expr{
+                    .unary = UnaryExpr{
+                        .operator = self.advance(),
+                        .right = try self.unary(),
+                    },
+                };
+                return expr;
+            },
+            else => return self.primary(),
+        }
+    }
+
+    fn primary(self: *Self) error{OutOfMemory}!*Expr {
+        var expr = try self.allocator.create(Expr);
+        const token = self.advance();
+        switch (token.token_type) {
+            TokenType.FALSE => expr.* = Expr{ .literal = LiteralExpr{ .boolean = false } },
+            TokenType.TRUE => expr.* = Expr{ .literal = LiteralExpr{ .boolean = true } },
+            TokenType.NIL => expr.* = Expr{ .literal = LiteralExpr{ .nil = false } },
+            TokenType.NUMBER => expr.* = Expr{ .literal = LiteralExpr{ .number = token.value.?.number } },
+            TokenType.STRING => expr.* = Expr{ .literal = LiteralExpr{ .string = token.value.?.string } },
+            TokenType.LEFT_PAREN => {
+                expr.* = Expr{
+                    .grouping = GroupingExpr{ .expr = try self.expression() },
+                };
+                var next_token = self.advance();
+                if (next_token.token_type != TokenType.RIGHT_PAREN) {
+                    err(next_token.line, "Unclosed left paren.");
+                }
+            },
+            else => {
+                err(token.line, "Unexpected primary token type.");
+                expr.* = Expr{ .literal = LiteralExpr{ .nil = false } };
+            },
+        }
+        return expr;
+    }
+
+    fn peekType(self: Self) TokenType {
+        return self.peek().token_type;
+    }
+
+    fn peek(self: Self) Token {
+        return self.tokens.items[self.current];
+    }
+
+    fn previous(self: Self) Token {
+        // FIXME: Bounds check.
+        return self.tokens.items[self.current - 1];
+    }
+
+    fn isAtEnd(self: Self) bool {
+        return self.peekType() == TokenType.EOF;
+    }
+
+    fn advance(self: *Self) Token {
+        if (self.isAtEnd()) {
+            return self.peek();
+        }
+        self.current += 1;
+        return self.previous();
+    }
+};
diff --git a/src/scanner.zig b/src/scanner.zig
index 0363b78..9a5b768 100644
--- a/src/scanner.zig
+++ b/src/scanner.zig
@@ -33,7 +33,7 @@ pub const Scanner = struct {
         }
 
         // FIXME: Handle error.
-        self.tokens.append(token.Token{ .token_type = token.TokenType.EOF, .lexeme = "", .line = self.line }) catch {};
+        self.tokens.append(token.Token{ .token_type = token.TokenType.EOF, .lexeme = "", .line = self.line, .value = null }) catch {};
         return self.tokens;
     }
 
@@ -156,7 +156,7 @@ pub const Scanner = struct {
             while (isDigit(self.peek())) _ = self.advance();
         }
 
-        self.addToken(token.TokenType.NUMBER);
+        self.addNumber();
     }
 
     fn identifier(self: *Scanner) void {
@@ -207,8 +207,18 @@ pub const Scanner = struct {
         return isDigit(char) or isAlpha(char);
     }
 
-    fn addToken(self: *Scanner, token_type: token.TokenType) void {
+    fn addTokenInternal(self: *Scanner, token_type: token.TokenType, token_value: ?token.Token.Value) void {
         // FIXME: Handle error.
-        self.tokens.append(token.Token{ .token_type = token_type, .lexeme = self.source[self.start..self.current], .line = self.line }) catch {};
+        self.tokens.append(token.Token{ .token_type = token_type, .lexeme = self.source[self.start..self.current], .line = self.line, .value = token_value }) catch {};
+    }
+
+    fn addToken(self: *Scanner, token_type: token.TokenType) void {
+        self.addTokenInternal(token_type, null);
+    }
+
+    fn addNumber(self: *Scanner) void {
+        // FIXME: Handle errors.
+        const float = std.fmt.parseFloat(f64, self.source[self.start..self.current]) catch 0;
+        self.addTokenInternal(token.TokenType.NUMBER, token.Token.Value{ .number = float });
     }
 };
diff --git a/src/token.zig b/src/token.zig
index 537a301..5f0cf80 100644
--- a/src/token.zig
+++ b/src/token.zig
@@ -54,6 +54,12 @@ pub const Token = struct {
     token_type: TokenType,
     lexeme: []const u8,
     line: u64,
+    value: ?Value,
+
+    pub const Value = union {
+        number: f64,
+        string: []const u8,
+    };
 
     fn toString(self: *Token, alloc: std.mem.Allocator) ![]u8 {
         return std.fmt.allocPrint(alloc, "{} {} {}", .{ self.token_type, self.lexeme, self.line });
diff --git a/test.lox b/test.lox
index 0ecbd83..b9f88a1 100644
--- a/test.lox
+++ b/test.lox
@@ -1 +1 @@
-print "hello world!";
+1 + 5 / 3 == -9