diff --git a/src/types/__tests__/visitor.spec-d.ts b/src/types/__tests__/visitor.spec-d.ts
new file mode 100644
index 0000000..1ee1b40
--- /dev/null
+++ b/src/types/__tests__/visitor.spec-d.ts
@@ -0,0 +1,62 @@
+/**
+ * @file Type Tests - Visitor
+ * @module unist-util-visit/types/tests/unit-d/Visitor
+ */
+
+import type * as docast from '@flex-development/docast'
+import type { Optional } from '@flex-development/tutils'
+import type {
+  Index,
+  MatchInclusiveDescendant,
+  Type
+} from '@flex-development/unist-util-types'
+import type VisitedAncestor from '../visited-ancestor'
+import type VisitedParent from '../visited-parent'
+import type TestSubject from '../visitor'
+import type VisitorResult from '../visitor-result'
+
+describe('unit-d:types/Visitor', () => {
+  type Tree = docast.Root
+  type Check = Type<docast.InlineTag>
+  type Subject = TestSubject<Tree, Check>
+
+  it('should match [this: void]', () => {
+    expectTypeOf<Subject>().thisParameter.toBeVoid()
+  })
+
+  describe('parameters', () => {
+    it('should match [0: MatchInclusiveDescendant<Tree, Check>]', () => {
+      // Arrange
+      type Expect = MatchInclusiveDescendant<Tree, Check>
+
+      // Expect
+      expectTypeOf<Subject>().parameter(0).toEqualTypeOf<Expect>()
+    })
+
+    it('should match [1: Optional<Index>]', () => {
+      expectTypeOf<Subject>().parameter(1).toEqualTypeOf<Optional<Index>>()
+    })
+
+    it('should match [2: Optional<VisitedParent<Tree, Check>>]', () => {
+      // Arrange
+      type Expect = Optional<VisitedParent<Tree, Check>>
+
+      // Expect
+      expectTypeOf<Subject>().parameter(2).toEqualTypeOf<Expect>()
+    })
+
+    it('should match [3: VisitedAncestor<Tree, Check>[]]', () => {
+      // Arrange
+      type Expect = VisitedAncestor<Tree, Check>[]
+
+      // Expect
+      expectTypeOf<Subject>().parameter(3).toEqualTypeOf<Expect>()
+    })
+  })
+
+  describe('returns', () => {
+    it('should return VisitorResult', () => {
+      expectTypeOf<Subject>().returns.toEqualTypeOf<VisitorResult>()
+    })
+  })
+})
diff --git a/src/types/__tests__/visitors.spec-d.ts b/src/types/__tests__/visitors.spec-d.ts
new file mode 100644
index 0000000..80f88d8
--- /dev/null
+++ b/src/types/__tests__/visitors.spec-d.ts
@@ -0,0 +1,29 @@
+/**
+ * @file Type Tests - Visitors
+ * @module unist-util-visit/types/tests/unit-d/Visitors
+ */
+
+import type * as docast from '@flex-development/docast'
+import type { Nilable } from '@flex-development/tutils'
+import type { Type } from '@flex-development/unist-util-types'
+import type Visitor from '../visitor'
+import type TestSubject from '../visitors'
+
+describe('unit-d:types/Visitors', () => {
+  type Tree = docast.Root
+  type Check = Type<docast.Comment>
+  type VisitorFn = Visitor<Tree, Check>
+  type Subject = TestSubject<Tree, Check>
+
+  it('should match [enter?: Nilable<Visitor<Tree, Check>>]', () => {
+    expectTypeOf<Subject>()
+      .toHaveProperty('enter')
+      .toEqualTypeOf<Nilable<VisitorFn>>()
+  })
+
+  it('should match [leave?: Nilable<Visitor<Tree, Check>>]', () => {
+    expectTypeOf<Subject>()
+      .toHaveProperty('leave')
+      .toEqualTypeOf<Nilable<VisitorFn>>()
+  })
+})
diff --git a/src/types/index.ts b/src/types/index.ts
index 2b47058..a64ea05 100644
--- a/src/types/index.ts
+++ b/src/types/index.ts
@@ -11,4 +11,6 @@ export type { default as Exit } from './exit'
 export type { default as Skip } from './skip'
 export type { default as VisitedAncestor } from './visited-ancestor'
 export type { default as VisitedParent } from './visited-parent'
+export type { default as Visitor } from './visitor'
 export type { default as VisitorResult } from './visitor-result'
+export type { default as Visitors } from './visitors'
diff --git a/src/types/visitor.ts b/src/types/visitor.ts
new file mode 100644
index 0000000..f2c38ca
--- /dev/null
+++ b/src/types/visitor.ts
@@ -0,0 +1,75 @@
+/**
+ * @file Type Definitions - Visitor
+ * @module unist-util-visit/types/Visitor
+ */
+
+import type { Optional } from '@flex-development/tutils'
+import type {
+  Index,
+  MatchInclusiveDescendant,
+  Test
+} from '@flex-development/unist-util-types'
+import type { Node } from 'unist'
+import type Action from './action'
+import type VisitedAncestor from './visited-ancestor'
+import type VisitedParent from './visited-parent'
+import type VisitorResult from './visitor-result'
+
+/**
+ * Handle visiting `node`.
+ *
+ * Visitors are free to transform `node`. They can also transform [`parent`][1],
+ * or the grandparent of `node` (the last of [`ancestors`][2]).
+ *
+ * > 👉 **Note**: Replacing `node` itself, if `SKIP` is not returned, still
+ * > causes its [*descendants*][3] to be walked (which is a bug).
+ *
+ * When adding or removing previous [*siblings*][4] of `node`, the `Visitor`
+ * should return a new `Index` to specify the sibling to traverse after `node`
+ * is traversed. Adding or removing next siblings of `node` is handled as
+ * expected without needing to return a new `Index`.
+ *
+ * Removing the [*children*][5] of an [*ancestor*][2] still results in those
+ * child nodes being traversed.
+ *
+ * The `Visitor` should return the next action, or `Index`, as explained above.
+ * An `Index` is treated as a tuple of `[CONTINUE, Index]`. An `Action` is
+ * treated as a tuple of `[Action]`. Returning a tuple only makes sense if the
+ * `Action` is `SKIP`. When the `Action` is `EXIT`, that action can be returned.
+ * When the `Action` is `CONTINUE`, `Index` can be returned.
+ *
+ * [1]: https://github.com/syntax-tree/unist#parent-1
+ * [2]: https://github.com/syntax-tree/unist#ancestor
+ * [3]: https://github.com/syntax-tree/unist#descendant
+ * [4]: https://github.com/syntax-tree/unist#sibling
+ * [5]: https://github.com/syntax-tree/unist#child
+ *
+ * @see {@linkcode Action}
+ * @see {@linkcode Index}
+ * @see {@linkcode MatchInclusiveDescendant}
+ * @see {@linkcode Node}
+ * @see {@linkcode Test}
+ * @see {@linkcode VisitedAncestor}
+ * @see {@linkcode VisitedParent}
+ * @see {@linkcode VisitorResult}
+ *
+ * @template {Node} [Tree=Node] - Tree being visited
+ * @template {Test} [Check=Test] - Node test
+ *
+ * @this {void}
+ *
+ * @param {MatchInclusiveDescendant<Tree, Check>} node - Found node
+ * @param {Optional<Index>} index - Index of `node` in `parent.children`
+ * @param {Optional<VisitedParent<Tree, Check>>} parent - Parent of `node`
+ * @param {VisitedAncestor<Tree, Check>[]} ancestors - Ancestors of `node`
+ * @return {VisitorResult} What to do next
+ */
+type Visitor<Tree extends Node = Node, Check extends Test = Test> = (
+  this: void,
+  node: MatchInclusiveDescendant<Tree, Check>,
+  index: Optional<Index>,
+  parent: Optional<VisitedParent<Tree, Check>>,
+  ancestors: VisitedAncestor<Tree, Check>[]
+) => VisitorResult
+
+export type { Visitor as default }
diff --git a/src/types/visitors.ts b/src/types/visitors.ts
new file mode 100644
index 0000000..2d61395
--- /dev/null
+++ b/src/types/visitors.ts
@@ -0,0 +1,43 @@
+/**
+ * @file Type Definitions - Visitors
+ * @module unist-util-visit/types/Visitors
+ */
+
+import type { Nilable } from '@flex-development/tutils'
+import type { Test } from '@flex-development/unist-util-types'
+import type { Node } from 'unist'
+import type Visitor from './visitor'
+
+/**
+ * Handle nodes when entering ([*preorder*][1]) and leaving ([*postorder*][2]).
+ *
+ * [1]: https://github.com/syntax-tree/unist#preorder
+ * [2]: https://github.com/syntax-tree/unist#postorder
+ *
+ * @see {@linkcode Node}
+ * @see {@linkcode Test}
+ *
+ * @template {Node} [Tree=Node] - Tree being visited
+ * @template {Test} [Check=Test] - Node test
+ */
+type Visitors<Tree extends Node = Node, Check extends Test = Test> = {
+  /**
+   * Handle nodes when entering ([*preorder*][1]).
+   *
+   * [1]: https://github.com/syntax-tree/unist#preorder
+   *
+   * @see {@linkcode Visitor}
+   */
+  enter?: Nilable<Visitor<Tree, Check>>
+
+  /**
+   * Handle nodes when leaving ([*postorder*][1]).
+   *
+   * [1]: https://github.com/syntax-tree/unist#postorder
+   *
+   * @see {@linkcode Visitor}
+   */
+  leave?: Nilable<Visitor<Tree, Check>>
+}
+
+export type { Visitors as default }