diff --git a/CLAUDE.md b/CLAUDE.md
index f4d8c1c..ba665c7 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -50,6 +50,11 @@ Transcript-based shell integration test runner. Bun + TypeScript.
   - Setup files can contain `@env` and `@teardown` directives but not `@setup` (no nesting)
   - User file `@env` overrides setup file `@env`
   - Setup command failures abort the test with an error
+- `@def name body` before first command = define a macro
+  - If a command matches `name` exactly, `body` is substituted before execution
+  - Backslash `\` at end of line continues the body onto the next line
+  - Allowed in both `.shout` files and setup files
+  - User file `@def` overrides setup file `@def` with the same name
 - Each file runs in a fresh temp dir with a single `/bin/sh` session
 - `$HOME` and `$SHOUT_DIR` are set to the temp dir automatically
 - `$SHOUT_SOURCE_DIR` is set to the directory containing the `.shout` file
diff --git a/README.md b/README.md
index 6ab7326..019261c 100644
--- a/README.md
+++ b/README.md
@@ -99,6 +99,29 @@ export DB_URL=sqlite:data/test.db
 @teardown rm -f "$SHOUT_PROJECT_DIR/data/test.db"
 ```
 
+### `@def`
+
+Define a macro that substitutes a command by name:
+
+```
+@def greet echo "hello world"
+
+$ greet
+hello world
+```
+
+Use backslash `\` for multi-line bodies:
+
+```
+@def serve \
+  python3 -m http.server $PORT & \
+  sleep 0.5
+
+$ serve
+```
+
+Macros defined in setup files are inherited. A user file `@def` with the same name overrides the setup version.
+
 ```
 Usage: shout test [options] [files...]
 
diff --git a/src/cli/index.ts b/src/cli/index.ts
index 8d6c963..a682613 100755
--- a/src/cli/index.ts
+++ b/src/cli/index.ts
@@ -117,6 +117,8 @@ program
       const envVars: Record<string, string> = {}
       const setupEnvVars: Record<string, string> = {}
       const userEnvVars: Record<string, string> = {}
+      const setupMacros: Record<string, string> = {}
+      const userMacros: Record<string, string> = {}
       const setupCommands: Command[] = []
       const teardownCommands: Command[] = [...parsed.teardownCommands]
       for (const d of parsed.directives) {
@@ -126,18 +128,33 @@ program
           const setupParsed = parseSetup(relative(cwd, setupPath), setupContent)
           for (const sd of setupParsed.directives) {
             if (sd.type === "env") setupEnvVars[sd.key] = sd.value
+            else if (sd.type === "def") setupMacros[sd.name] = sd.body
           }
           setupCommands.push(...setupParsed.commands)
           teardownCommands.push(...setupParsed.teardownCommands)
         } else if (d.type === "env") {
           userEnvVars[d.key] = d.value
+        } else if (d.type === "def") {
+          userMacros[d.name] = d.body
         }
       }
       Object.assign(envVars, setupEnvVars, userEnvVars)
       if (!("PORT" in userEnvVars) && !("PORT" in setupEnvVars)) {
         envVars["PORT"] = String(port)
       }
-      const merged: ShoutFile = { ...parsed, commands: [...setupCommands, ...parsed.commands, ...teardownCommands] }
+      const macros: Record<string, string> = { ...setupMacros, ...userMacros }
+      const expandMacro = (cmd: Command): Command => {
+        const body = macros[cmd.command]
+        return body !== undefined ? { ...cmd, command: body } : cmd
+      }
+      const merged: ShoutFile = {
+        ...parsed,
+        commands: [
+          ...setupCommands.map(expandMacro),
+          ...parsed.commands.map(expandMacro),
+          ...teardownCommands.map(expandMacro),
+        ],
+      }
 
       const setupLen = setupCommands.length
       const userLen = parsed.commands.length
diff --git a/src/parse.test.ts b/src/parse.test.ts
index f4941f8..0fa7c98 100644
--- a/src/parse.test.ts
+++ b/src/parse.test.ts
@@ -185,6 +185,44 @@ describe("parse", () => {
       "test.shout:1: unknown directive: @evn PORT=3000",
     )
   })
+
+  test("@def simple macro", () => {
+    const result = parse("test.shout", "@def greet echo hello\n$ greet\nhello\n")
+    expect(result.directives).toEqual([
+      { type: "def", name: "greet", body: "echo hello", line: 1 },
+    ])
+    expect(result.commands).toHaveLength(1)
+    expect(result.commands[0]!.command).toBe("greet")
+  })
+
+  test("@def with backslash continuation", () => {
+    const content = "@def multi echo one; \\\n  echo two\n$ multi\none\ntwo\n"
+    const result = parse("test.shout", content)
+    expect(result.directives).toEqual([
+      { type: "def", name: "multi", body: "echo one;\necho two", line: 1 },
+    ])
+    expect(result.commands).toHaveLength(1)
+  })
+
+  test("@def multiple macros", () => {
+    const content = "@def foo echo foo\n@def bar echo bar\n$ foo\nfoo\n"
+    const result = parse("test.shout", content)
+    expect(result.directives).toHaveLength(2)
+    expect(result.directives[0]).toEqual({ type: "def", name: "foo", body: "echo foo", line: 1 })
+    expect(result.directives[1]).toEqual({ type: "def", name: "bar", body: "echo bar", line: 2 })
+  })
+
+  test("@def without body throws", () => {
+    expect(() => parse("test.shout", "@def greet\n$ echo hi\n")).toThrow(
+      "test.shout:1: @def requires a name and body",
+    )
+  })
+
+  test("@def after first command is expected output", () => {
+    const result = parse("test.shout", "$ cat file\n@def foo bar\n")
+    expect(result.directives).toEqual([])
+    expect(result.commands[0]!.expected).toEqual(["@def foo bar"])
+  })
 })
 
 describe("parseSetup", () => {
@@ -252,4 +290,19 @@ describe("parseSetup", () => {
     expect(result.commands[0]!.expected).toEqual([])
     expect(result.commands[0]!.exitCode).toBeNull()
   })
+
+  test("@def in setup file", () => {
+    const result = parseSetup("setup.shout", "@def greet echo hello\nexport FOO=bar\n")
+    expect(result.directives).toEqual([
+      { type: "def", name: "greet", body: "echo hello", line: 1 },
+    ])
+    expect(result.commands).toHaveLength(1)
+  })
+
+  test("@def with backslash continuation in setup file", () => {
+    const result = parseSetup("setup.shout", "@def multi echo one; \\\n  echo two\n")
+    expect(result.directives).toEqual([
+      { type: "def", name: "multi", body: "echo one;\necho two", line: 1 },
+    ])
+  })
 })
diff --git a/src/parse.ts b/src/parse.ts
index b5709bc..413588e 100644
--- a/src/parse.ts
+++ b/src/parse.ts
@@ -11,6 +11,7 @@ export type Command = {
 export type Directive =
   | { type: "setup"; path: string; line: number }
   | { type: "env"; key: string; value: string; line: number }
+  | { type: "def"; name: string; body: string; line: number }
 
 export type ShoutFile = {
   path: string
@@ -65,6 +66,32 @@ function parseEnvDirective(path: string, line: string, lineNum: number): { key:
   return { key: rest.slice(0, eq), value: rest.slice(eq + 1) }
 }
 
+function parseDefDirective(
+  path: string,
+  rawLines: string[],
+  i: number,
+): { name: string; body: string; linesConsumed: number } {
+  const rest = rawLines[i]!.slice(5).trim() // strip "@def "
+  const spaceIdx = rest.indexOf(" ")
+  if (spaceIdx < 0) {
+    throw new Error(`${path}:${i + 1}: @def requires a name and body`)
+  }
+  const name = rest.slice(0, spaceIdx)
+  let body = rest.slice(spaceIdx + 1)
+  let extra = 0
+
+  while (body.endsWith("\\") && i + extra + 1 < rawLines.length) {
+    body = body.slice(0, -1).trimEnd() + "\n" + rawLines[i + extra + 1]!.trim()
+    extra++
+  }
+
+  if (!body.trim()) {
+    throw new Error(`${path}:${i + 1}: @def requires a name and body`)
+  }
+
+  return { name, body, linesConsumed: extra }
+}
+
 export function parseSetup(path: string, content: string): ShoutFile {
   const rawLines = content.split("\n")
 
@@ -97,6 +124,10 @@ export function parseSetup(path: string, content: string): ShoutFile {
           expected: [],
           exitCode: null,
         })
+      } else if (line.startsWith("@def ")) {
+        const { name, body, linesConsumed } = parseDefDirective(path, rawLines, i)
+        directives.push({ type: "def", name, body, line: i + 1 })
+        i += linesConsumed
       } else if (line.startsWith("@setup ")) {
         throw new Error(`${path}:${i + 1}: @setup not allowed in setup files`)
       } else {
@@ -156,6 +187,10 @@ export function parse(path: string, content: string): ShoutFile {
       } else if (line.startsWith("@env ")) {
         const { key, value } = parseEnvDirective(path, line, i + 1)
         directives.push({ type: "env", key, value, line: i + 1 })
+      } else if (line.startsWith("@def ")) {
+        const { name, body, linesConsumed } = parseDefDirective(path, rawLines, i)
+        directives.push({ type: "def", name, body, line: i + 1 })
+        i += linesConsumed
       } else {
         throw new Error(`${path}:${i + 1}: unknown directive: ${line}`)
       }
diff --git a/test/def-override.shout b/test/def-override.shout
new file mode 100644
index 0000000..dcf4ca1
--- /dev/null
+++ b/test/def-override.shout
@@ -0,0 +1,5 @@
+@setup def-shared.shout
+@def serve echo "overridden"
+
+$ serve
+overridden
diff --git a/test/def-setup.shout b/test/def-setup.shout
new file mode 100644
index 0000000..227c2cc
--- /dev/null
+++ b/test/def-setup.shout
@@ -0,0 +1,4 @@
+@setup def-shared.shout
+
+$ serve
+server started
diff --git a/test/def-shared.shout b/test/def-shared.shout
new file mode 100644
index 0000000..c0e12b2
--- /dev/null
+++ b/test/def-shared.shout
@@ -0,0 +1 @@
+@def serve echo "server started"
diff --git a/test/def.shout b/test/def.shout
new file mode 100644
index 0000000..08324d5
--- /dev/null
+++ b/test/def.shout
@@ -0,0 +1,13 @@
+@def greet echo "hello world"
+@def multi echo "line one"; \
+  echo "line two"
+
+$ greet
+hello world
+
+$ multi
+line one
+line two
+
+$ echo "not a macro"
+not a macro
diff --git a/web/index.html b/web/index.html
index 8fa1ab2..e7761f6 100644
--- a/web/index.html
+++ b/web/index.html
@@ -218,6 +218,16 @@
   <p><code>@teardown</code> can appear in both <code>.shout</code> files and setup files. Teardown failures produce warnings but don't affect test results.</p>
 </section>
 
+<section>
+  <h2>Macros</h2>
+  <p>Use <code class="bright">@def</code> to define reusable command macros.</p>
+  <pre><code><span class="bright">@def</span> <span class="cmd">greet echo "hello world"</span>
+
+<span class="prompt">$</span> <span class="cmd">greet</span>
+<span class="output">hello world</span></code></pre>
+  <p>If a command matches a macro name exactly, the body is substituted. Use <code>\</code> for multi-line bodies. Macros from setup files are inherited; user-file macros override them.</p>
+</section>
+
 <section>
   <h2>Run it</h2>
   <pre><code><span class="prompt">$</span> <span class="cmd">shout test</span>