diff --git a/src/Chip8.Core/Assembly/MacroExpander.cs b/src/Chip8.Core/Assembly/MacroExpander.cs
new file mode 100644
index 0000000..1d936b8
--- /dev/null
+++ b/src/Chip8.Core/Assembly/MacroExpander.cs
@@ -0,0 +1,225 @@
+using System;
+using System.Collections.Generic;
+
+namespace Chip8.Core.Assembly;
+
+public sealed class MacroExpander
+{
+    const int expansionDepthLimit = 256;
+
+    sealed class MacroDefinition
+    {
+        public string name = "";
+        public List<string> parameters = new();
+        public List<List<Token>> body = new();
+        public SourcePosition position;
+    }
+
+    readonly Dictionary<string, MacroDefinition> macros = new(StringComparer.OrdinalIgnoreCase);
+
+    public List<Token> expand(List<Token> tokens)
+    {
+        List<List<Token>> lines = splitLines(tokens);
+        List<List<Token>> withoutDefinitions = collectDefinitions(lines);
+        var output = new List<List<Token>>();
+        emitLines(withoutDefinitions, output, 0);
+        return joinLines(output);
+    }
+
+    List<List<Token>> collectDefinitions(List<List<Token>> lines)
+    {
+        var remaining = new List<List<Token>>();
+        int index = 0;
+
+        while (index < lines.Count)
+        {
+            List<Token> line = lines[index];
+            if (line.Count > 0 && line[0].isIdentifier("MACRO"))
+            {
+                index = readDefinition(lines, index);
+            }
+            else
+            {
+                remaining.Add(line);
+                index++;
+            }
+        }
+
+        return remaining;
+    }
+
+    int readDefinition(List<List<Token>> lines, int index)
+    {
+        List<Token> header = lines[index];
+        if (header.Count < 2)
+        {
+            throw new AsmException("MACRO needs a name", header[0].position);
+        }
+
+        var definition = new MacroDefinition { name = header[1].text, position = header[0].position };
+        for (int p = 2; p < header.Count; p++)
+        {
+            if (header[p].kind == TokenKind.Comma)
+            {
+                continue;
+            }
+            if (header[p].kind != TokenKind.Identifier)
+            {
+                throw new AsmException("macro parameter must be a name", header[p].position);
+            }
+            definition.parameters.Add(header[p].text);
+        }
+
+        int cursor = index + 1;
+        while (cursor < lines.Count)
+        {
+            List<Token> line = lines[cursor];
+            if (line.Count > 0 && (line[0].isIdentifier("ENDM") || line[0].isIdentifier("ENDMACRO")))
+            {
+                macros[definition.name] = definition;
+                return cursor + 1;
+            }
+            definition.body.Add(line);
+            cursor++;
+        }
+
+        throw new AsmException("macro '" + definition.name + "' is missing ENDM", definition.position);
+    }
+
+    void emitLines(List<List<Token>> lines, List<List<Token>> output, int depth)
+    {
+        if (depth > expansionDepthLimit)
+        {
+            throw new AsmException("macro expansion too deep (recursive macro?)", lines.Count > 0 && lines[0].Count > 0 ? lines[0][0].position : new SourcePosition(0, 0));
+        }
+
+        foreach (List<Token> original in lines)
+        {
+            List<Token> line = original;
+
+            if (line.Count >= 2 && line[0].kind == TokenKind.Identifier && line[1].kind == TokenKind.Colon)
+            {
+                output.Add(new List<Token> { line[0], line[1] });
+                line = line.GetRange(2, line.Count - 2);
+            }
+
+            if (line.Count > 0 && line[0].kind == TokenKind.Identifier && macros.TryGetValue(line[0].text, out MacroDefinition? definition))
+            {
+                List<List<Token>> expanded = instantiate(definition, line);
+                emitLines(expanded, output, depth + 1);
+            }
+            else if (line.Count > 0)
+            {
+                output.Add(line);
+            }
+        }
+    }
+
+    static List<List<Token>> instantiate(MacroDefinition definition, List<Token> call)
+    {
+        List<List<Token>> arguments = splitArguments(call, 1);
+        if (arguments.Count != definition.parameters.Count)
+        {
+            throw new AsmException(
+                "macro '" + definition.name + "' expects " + definition.parameters.Count + " argument(s), got " + arguments.Count,
+                call[0].position);
+        }
+
+        var binding = new Dictionary<string, List<Token>>(StringComparer.OrdinalIgnoreCase);
+        for (int p = 0; p < definition.parameters.Count; p++)
+        {
+            binding[definition.parameters[p]] = arguments[p];
+        }
+
+        var result = new List<List<Token>>();
+        foreach (List<Token> bodyLine in definition.body)
+        {
+            var substituted = new List<Token>();
+            foreach (Token token in bodyLine)
+            {
+                if (token.kind == TokenKind.Identifier && binding.TryGetValue(token.text, out List<Token>? replacement))
+                {
+                    substituted.AddRange(replacement);
+                }
+                else
+                {
+                    substituted.Add(token);
+                }
+            }
+            result.Add(substituted);
+        }
+
+        return result;
+    }
+
+    static List<List<Token>> splitArguments(List<Token> line, int start)
+    {
+        var arguments = new List<List<Token>>();
+        var current = new List<Token>();
+        int depth = 0;
+
+        for (int index = start; index < line.Count; index++)
+        {
+            Token token = line[index];
+            if (token.kind == TokenKind.LeftParen || token.kind == TokenKind.LeftBracket)
+            {
+                depth++;
+            }
+            else if (token.kind == TokenKind.RightParen || token.kind == TokenKind.RightBracket)
+            {
+                depth--;
+            }
+
+            if (token.kind == TokenKind.Comma && depth == 0)
+            {
+                arguments.Add(current);
+                current = new List<Token>();
+                continue;
+            }
+
+            current.Add(token);
+        }
+
+        if (current.Count > 0)
+        {
+            arguments.Add(current);
+        }
+
+        return arguments;
+    }
+
+    static List<List<Token>> splitLines(List<Token> tokens)
+    {
+        var lines = new List<List<Token>>();
+        var current = new List<Token>();
+
+        foreach (Token token in tokens)
+        {
+            if (token.kind == TokenKind.EndOfLine || token.kind == TokenKind.EndOfFile)
+            {
+                if (current.Count > 0)
+                {
+                    lines.Add(current);
+                    current = new List<Token>();
+                }
+                continue;
+            }
+            current.Add(token);
+        }
+
+        return lines;
+    }
+
+    static List<Token> joinLines(List<List<Token>> lines)
+    {
+        var tokens = new List<Token>();
+        foreach (List<Token> line in lines)
+        {
+            tokens.AddRange(line);
+            SourcePosition position = line.Count > 0 ? line[line.Count - 1].position : new SourcePosition(0, 0);
+            tokens.Add(new Token(TokenKind.EndOfLine, "\\n", 0, position));
+        }
+        tokens.Add(new Token(TokenKind.EndOfFile, "", 0, new SourcePosition(0, 0)));
+        return tokens;
+    }
+}