diff --git a/codebasin/preprocessor.py b/codebasin/preprocessor.py
index 03c189c..43234ec 100644
--- a/codebasin/preprocessor.py
+++ b/codebasin/preprocessor.py
@@ -11,8 +11,9 @@
 import hashlib
 import logging
 import os
-from collections.abc import Iterable
+from collections.abc import Callable, Iterable
 from copy import copy
+from enum import Enum
 from typing import Self
 
 import numpy as np
@@ -539,6 +540,11 @@ class ParseError(ValueError):
     """
 
 
+class Visit(Enum):
+    NEXT = 0
+    NEXT_SIBLING = 1
+
+
 class Node:
     """
     Base class for all other Node types.
@@ -597,6 +603,22 @@ def walk(self) -> Iterable[Self]:
         for child in self.children:
             yield from child.walk()
 
+    def visit(self, visitor: Callable[[Self], Visit]):
+        """
+        Visit all descendants of this node via a preorder traversal, using the
+        supplied visitor.
+
+        Raises
+        ------
+        TypeError
+            If `visitor` is not callable.
+        """
+        if not callable(visitor):
+            raise TypeError("visitor is not callable.")
+        if visitor(self) != Visit.NEXT_SIBLING:
+            for child in self.children:
+                child.visit(visitor)
+
 
 class FileNode(Node):
     """
@@ -2359,6 +2381,18 @@ def walk(self) -> Iterable[Node]:
         """
         yield from self.root.walk()
 
+    def visit(self, visitor: Callable[[Node], Visit]):
+        """
+        Visit each node in the tree via a preorder traversal, using the
+        supplied visitor.
+
+        Raises
+        ------
+        TypeError
+            If `visitor` is not callable.
+        """
+        self.root.visit(visitor)
+
     def associate_file(self, filename):
         self.root.filename = filename
 
diff --git a/tests/source-tree/test_source_tree.py b/tests/source-tree/test_source_tree.py
index 4d9463a..c47c0b9 100644
--- a/tests/source-tree/test_source_tree.py
+++ b/tests/source-tree/test_source_tree.py
@@ -7,7 +7,7 @@
 import warnings
 
 from codebasin.file_parser import FileParser
-from codebasin.preprocessor import CodeNode, DirectiveNode, FileNode
+from codebasin.preprocessor import CodeNode, DirectiveNode, FileNode, Visit
 
 
 class TestSourceTree(unittest.TestCase):
@@ -19,9 +19,6 @@ def setUp(self):
         logging.getLogger("codebasin").disabled = False
         warnings.simplefilter("ignore", ResourceWarning)
 
-    def test_walk(self):
-        """Check that walk() visits nodes in the expected order"""
-
         # TODO: Revisit this when SourceTree can be built without a file.
         with tempfile.NamedTemporaryFile(
             mode="w",
@@ -43,36 +40,94 @@ def test_walk(self):
             f.close()
 
             # TODO: Revisit this when __str__() is more reliable.
-            tree = FileParser(f.name).parse_file(summarize_only=False)
-            expected_types = [
-                FileNode,
-                DirectiveNode,
-                CodeNode,
-                DirectiveNode,
-                CodeNode,
-                DirectiveNode,
-                CodeNode,
-                DirectiveNode,
-                CodeNode,
-            ]
-            expected_contents = [
-                f.name,
-                "FOO",
-                "foo",
-                "BAR",
-                "bar",
-                "else",
-                "baz",
-                "endif",
-                "qux",
-            ]
-            for i, node in enumerate(tree.walk()):
-                self.assertTrue(isinstance(node, expected_types[i]))
-                if isinstance(node, CodeNode):
-                    contents = node.spelling()[0]
-                else:
-                    contents = str(node)
-                self.assertTrue(expected_contents[i] in contents)
+            self.tree = FileParser(f.name).parse_file(summarize_only=False)
+            self.filename = f.name
+
+    def test_walk(self):
+        """Check that walk() visits nodes in the expected order"""
+        expected_types = [
+            FileNode,
+            DirectiveNode,
+            CodeNode,
+            DirectiveNode,
+            CodeNode,
+            DirectiveNode,
+            CodeNode,
+            DirectiveNode,
+            CodeNode,
+        ]
+        expected_contents = [
+            self.filename,
+            "FOO",
+            "foo",
+            "BAR",
+            "bar",
+            "else",
+            "baz",
+            "endif",
+            "qux",
+        ]
+        for i, node in enumerate(self.tree.walk()):
+            self.assertTrue(isinstance(node, expected_types[i]))
+            if isinstance(node, CodeNode):
+                contents = node.spelling()[0]
+            else:
+                contents = str(node)
+            self.assertTrue(expected_contents[i] in contents)
+
+    def test_visit_types(self):
+        """Check that visit() validates inputs"""
+
+        class valid_visitor:
+            def __call__(self, node):
+                return True
+
+        self.tree.visit(valid_visitor())
+
+        def visitor_function(node):
+            return True
+
+        self.tree.visit(visitor_function)
+
+        with self.assertRaises(TypeError):
+            self.tree.visit(1)
+
+        class invalid_visitor:
+            pass
+
+        with self.assertRaises(TypeError):
+            self.tree.visit(invalid_visitor())
+
+    def test_visit(self):
+        """Check that visit() visits nodes as expected"""
+
+        # Check that a trivial visitor visits all nodes.
+        class NodeCounter:
+            def __init__(self):
+                self.count = 0
+
+            def __call__(self, node):
+                self.count += 1
+
+        node_counter = NodeCounter()
+        self.tree.visit(node_counter)
+        self.assertEqual(node_counter.count, 9)
+
+        # Check that returning NEXT_SIBLING prevents descent.
+        class TopLevelCounter:
+            def __init__(self):
+                self.count = 0
+
+            def __call__(self, node):
+                if not isinstance(node, FileNode):
+                    self.count += 1
+                if isinstance(node, DirectiveNode):
+                    return Visit.NEXT_SIBLING
+                return Visit.NEXT
+
+        top_level_counter = TopLevelCounter()
+        self.tree.visit(top_level_counter)
+        self.assertEqual(top_level_counter.count, 5)
 
 
 if __name__ == "__main__":