diff --git a/.gitignore b/.gitignore
index fcb26070..69d75b1a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,3 +5,4 @@ node_modules/
 *.log
 yarn.lock
 !/index.d.ts
+!/test/types.d.ts
diff --git a/index.d.ts b/index.d.ts
index ffb08843..e2d124a9 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -7,6 +7,7 @@ export type {
   Compiler,
   CompilerClass,
   CompilerFunction,
+  // `Data` is typed and exposed below.
   Pluggable,
   PluggableList,
   Plugin,
@@ -18,6 +19,7 @@ export type {
   ProcessCallback,
   Processor,
   RunCallback,
+  // `Settings` is typed and exposed below.
   TransformCallback,
   Transformer
 } from './lib/index.js'
@@ -40,6 +42,8 @@ export {unified} from './lib/index.js'
  *     ReactNode: ReactNode
  *   }
  * }
+ *
+ * export {} // You may not need this, but it makes sure the file is a module.
  * ```
  *
  * Use {@link CompileResults `CompileResults`} to access the values.
@@ -49,3 +53,54 @@ export interface CompileResultMap {
   Uint8Array: Uint8Array
   string: string
 }
+
+/**
+ * Interface of known data that can be supported by all plugins.
+ *
+ * Typically, options can be given to a specific plugin, but sometimes it makes
+ * sense to have information shared with several plugins.
+ * For example, a list of HTML elements that are self-closing, which is needed
+ * during all phases.
+ *
+ * To type this, do something like:
+ *
+ * ```ts
+ * declare module 'unified' {
+ *   interface Data {
+ *     htmlVoidElements?: Array<string> | undefined
+ *   }
+ * }
+ *
+ * export {} // You may not need this, but it makes sure the file is a module.
+ * ```
+ */
+export interface Data {
+  settings?: Settings | undefined
+}
+
+/**
+ * Interface of known extra options, that can be supported by parser and
+ * compilers.
+ *
+ * This exists so that users can use packages such as `remark`, which configure
+ * both parsers and compilers (in this case `remark-parse` and
+ * `remark-stringify`), and still provide options for them.
+ *
+ * When you make parsers or compilers, that could be packaged up together,
+ * you should support `this.data('settings')` as input and merge it with
+ * explicitly passed `options`.
+ * Then, to type it, using `remark-stringify` as an example, do something like:
+ *
+ * ```ts
+ * declare module 'unified' {
+ *   interface Settings {
+ *     bullet: '*' | '+' | '-'
+ *     // …
+ *   }
+ * }
+ *
+ * export {} // You may not need this, but it makes sure the file is a module.
+ * ```
+ */
+// eslint-disable-next-line @typescript-eslint/no-empty-interface
+export interface Settings {}
diff --git a/lib/index.js b/lib/index.js
index 0fcc16a5..9dbeec78 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -7,6 +7,7 @@
  * @typedef {import('vfile').VFileValue} VFileValue
  *
  * @typedef {import('../index.js').CompileResultMap} CompileResultMap
+ * @typedef {import('../index.js').Data} Data
  */
 
 /**
@@ -518,7 +519,7 @@ export class Processor extends CallableInstance {
      *
      * @deprecated
      *   This is a private internal property and should not be used.
-     * @type {{settings?: Record<string, unknown>} & Record<string, unknown>}
+     * @type {Data}
      */
     this.namespace = {}
 
@@ -573,26 +574,28 @@ export class Processor extends CallableInstance {
    * > 👉 **Note**: setting information cannot occur on *frozen* processors.
    * > Call the processor first to create a new unfrozen processor.
    *
+   * @template {keyof Data} Key
+   *
    * @overload
-   * @returns {Record<string, unknown>}
+   * @returns {Data}
    *
    * @overload
-   * @param {Record<string, unknown>} dataset
+   * @param {Data} dataset
    * @returns {Processor<ParseTree, HeadTree, TailTree, CompileTree, CompileResult>}
    *
    * @overload
-   * @param {string} key
-   * @returns {unknown}
+   * @param {Key} key
+   * @returns {Data[Key]}
    *
    * @overload
-   * @param {string} key
-   * @param {unknown} value
+   * @param {Key} key
+   * @param {Data[Key]} value
    * @returns {Processor<ParseTree, HeadTree, TailTree, CompileTree, CompileResult>}
    *
-   * @param {Record<string, unknown> | string} [key]
+   * @param {Data | Key} [key]
    *   Key to get or set, or entire dataset to set, or nothing to get the
    *   entire dataset (optional).
-   * @param {unknown} [value]
+   * @param {Data[Key]} [value]
    *   Value to set (optional).
    * @returns {unknown}
    *   The processor that `data` is called on when settings, the value at `key`
diff --git a/test/data.js b/test/data.js
index 671bdaab..6b8df6c2 100644
--- a/test/data.js
+++ b/test/data.js
@@ -18,6 +18,8 @@ test('`data`', async function (t) {
   })
 
   await t.test('should not yield data prototypal fields', async function () {
+    // @ts-expect-error: `toString` is not a typed key of `Data`.
+    // But it exists on objects, so we test that here.
     assert.equal(unified().data('toString'), undefined)
   })
 
diff --git a/test/types.d.ts b/test/types.d.ts
new file mode 100644
index 00000000..5c293242
--- /dev/null
+++ b/test/types.d.ts
@@ -0,0 +1,9 @@
+declare module 'unified' {
+  interface Data {
+    baz?: 'qux' | undefined
+    foo?: 'bar' | undefined
+    x?: boolean | undefined
+  }
+}
+
+export {} // You may not need this, but it makes sure the file is a module.
diff --git a/tsconfig.json b/tsconfig.json
index ad1496e9..8e70d195 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -11,5 +11,5 @@
     "target": "es2020"
   },
   "exclude": ["coverage/", "node_modules/"],
-  "include": ["**/*.js", "index.d.ts"]
+  "include": ["**/*.js", "test/types.d.ts", "index.d.ts"]
 }