diff --git a/packages/ui/src/core.ts b/packages/ui/src/core.ts
index cb0ff5c..821b7f6 100644
--- a/packages/ui/src/core.ts
+++ b/packages/ui/src/core.ts
@@ -1 +1,2 @@
-export {};
+export * from './core/context.ts';
+export * from './core/node.ts';
diff --git a/packages/ui/src/core/context.test.ts b/packages/ui/src/core/context.test.ts
new file mode 100644
index 0000000..97b24de
--- /dev/null
+++ b/packages/ui/src/core/context.test.ts
@@ -0,0 +1,182 @@
+import { describe, expect, test } from 'vitest';
+import { createContext, MissingContextError } from './context.ts';
+import { Node } from './node.ts';
+
+const theme = createContext<string>('theme');
+const count = createContext<number>('count');
+const locale = createContext<string>('locale', 'en');
+
+describe('context', () => {
+	describe('on a single node', () => {
+		test('get returns undefined for a context that was never set', () => {
+			expect(theme.get(new Node())).toBeUndefined();
+		});
+
+		test('does not inherit values from Object.prototype', () => {
+			const constructor = createContext('constructor');
+
+			expect(constructor.get(new Node())).toBeUndefined();
+		});
+
+		test('get returns the value after it is set', () => {
+			const n = new Node();
+			theme.set(n, 'dark');
+			expect(theme.get(n)).toBe('dark');
+		});
+
+		test('the last set wins', () => {
+			const n = new Node();
+			theme.set(n, 'dark');
+			theme.set(n, 'light');
+			expect(theme.get(n)).toBe('light');
+		});
+
+		test('keeps falsy values — 0 is stored, not treated as unset', () => {
+			const n = new Node();
+			count.set(n, 0);
+			expect(count.get(n)).toBe(0);
+		});
+
+		test('distinct contexts do not collide', () => {
+			const n = new Node();
+			theme.set(n, 'dark');
+			expect(count.get(n)).toBeUndefined();
+		});
+
+		test('contexts with the same name share a value', () => {
+			const first = createContext<string>('shared');
+			const second = createContext<string>('shared');
+			const n = new Node();
+			first.set(n, 'value');
+			expect(second.get(n)).toBe('value');
+		});
+
+		test('expect returns the value when the context is set', () => {
+			const n = new Node();
+			theme.set(n, 'dark');
+			expect(theme.expect(n)).toBe('dark');
+		});
+
+		test('expect throws MissingContextError when unset with no default', () => {
+			expect(() => theme.expect(new Node())).toThrow(MissingContextError);
+		});
+
+		test('expect falls back to the default when unset', () => {
+			expect(locale.expect(new Node())).toBe('en');
+		});
+
+		test('a set value takes precedence over the default', () => {
+			const n = new Node();
+			locale.set(n, 'fr');
+			expect(locale.expect(n)).toBe('fr');
+		});
+
+		test('get returns the default when the context is unset', () => {
+			expect(locale.get(new Node())).toBe('en');
+		});
+
+		test('a set value overrides the default for get', () => {
+			const n = new Node();
+			locale.set(n, 'fr');
+			expect(locale.get(n)).toBe('fr');
+		});
+
+		test('hasOwn is false before a value is set', () => {
+			expect(theme.hasOwn(new Node())).toBe(false);
+		});
+
+		test('hasOwn is true once a value is set on the node', () => {
+			const n = new Node();
+			theme.set(n, 'dark');
+			expect(theme.hasOwn(n)).toBe(true);
+		});
+	});
+
+	describe('inheritance through the node tree', () => {
+		test('a child inherits a value set on its parent', () => {
+			const parent = new Node();
+			theme.set(parent, 'dark');
+			expect(theme.get(new Node(parent))).toBe('dark');
+		});
+
+		test('a value set on a child shadows the parent for that child only', () => {
+			const parent = new Node();
+			theme.set(parent, 'dark');
+			const child = new Node(parent);
+			theme.set(child, 'light');
+			expect(theme.get(child)).toBe('light');
+			expect(theme.get(parent)).toBe('dark');
+		});
+
+		test('setting a value on a child does not leak up to the parent', () => {
+			const parent = new Node();
+			const child = new Node(parent);
+			theme.set(child, 'light');
+			expect(theme.get(parent)).toBeUndefined();
+		});
+
+		test('expect inherits an ancestor value', () => {
+			const parent = new Node();
+			theme.set(parent, 'dark');
+			expect(theme.expect(new Node(parent))).toBe('dark');
+		});
+
+		test('an inherited value resolves through multiple levels', () => {
+			const grand = new Node();
+			theme.set(grand, 'dark');
+			const parent = new Node(grand);
+			expect(theme.get(new Node(parent))).toBe('dark');
+		});
+
+		test('an explicitly set undefined shadows the ancestor value', () => {
+			const parent = new Node();
+			locale.set(parent, 'fr');
+			const child = new Node(parent);
+			locale.set(child, undefined as unknown as string);
+			expect(locale.get(child)).toBe('en');
+			expect(locale.get(parent)).toBe('fr');
+		});
+
+		test('an explicitly set undefined shadows with no default to fall back to', () => {
+			const parent = new Node();
+			theme.set(parent, 'dark');
+			const child = new Node(parent);
+			theme.set(child, undefined as unknown as string);
+			expect(theme.get(child)).toBeUndefined();
+			expect(() => theme.expect(child)).toThrow(MissingContextError);
+		});
+
+		test('hasOwn is true for an explicitly set undefined', () => {
+			const parent = new Node();
+			theme.set(parent, 'dark');
+			const child = new Node(parent);
+			theme.set(child, undefined as unknown as string);
+			expect(theme.hasOwn(child)).toBe(true);
+		});
+
+		test('hasOwn is false for a value that is only inherited', () => {
+			const parent = new Node();
+			theme.set(parent, 'dark');
+			expect(theme.hasOwn(new Node(parent))).toBe(false);
+		});
+
+		test('hasOwn is true only for the node that owns the value', () => {
+			const parent = new Node();
+			theme.set(parent, 'dark');
+			const child = new Node(parent);
+			theme.set(child, 'light');
+			expect(theme.hasOwn(child)).toBe(true);
+			expect(theme.hasOwn(parent)).toBe(true);
+		});
+
+		test('expect falls back to the default when neither node nor ancestors set it', () => {
+			const parent = new Node();
+			expect(locale.expect(new Node(parent))).toBe('en');
+		});
+
+		test('get falls back to the default when neither node nor ancestors set it', () => {
+			const parent = new Node();
+			expect(locale.get(new Node(parent))).toBe('en');
+		});
+	});
+});
diff --git a/packages/ui/src/core/context.ts b/packages/ui/src/core/context.ts
new file mode 100644
index 0000000..536764b
--- /dev/null
+++ b/packages/ui/src/core/context.ts
@@ -0,0 +1,67 @@
+import type { Node } from './node.ts';
+
+/**
+ * A `Context` defines a value which is in effect for a given {@link Node}.
+ *
+ * Unless a context value is set for a particular node, it will inherit its
+ * value from that node's parent.
+ */
+export class Context<T> {
+	/** A unique identifier for this context. */
+	readonly name: string;
+
+	/** The value returned when this context is not present on a node. */
+	readonly defaultValue?: T;
+
+	constructor(name: string, defaultValue?: T) {
+		this.name = name;
+		this.defaultValue = defaultValue;
+	}
+
+	get(node: Node): T | undefined {
+		for (let n: Node | undefined = node; n; n = n.parent) {
+			const contexts = n.contexts;
+			if (contexts?.has(this.name)) {
+				const value = contexts.get(this.name) as T | undefined;
+				return value === undefined ? this.defaultValue : value;
+			}
+		}
+		return this.defaultValue;
+	}
+
+	set(node: Node, value: T): void {
+		const contexts = (node.contexts ??= new Map());
+		contexts.set(this.name, value);
+	}
+
+	expect(node: Node): T {
+		const value = this.get(node);
+		if (value === undefined) {
+			throw new MissingContextError(this.name);
+		} else {
+			return value;
+		}
+	}
+
+	hasOwn(node: Node): boolean {
+		return node.contexts?.has(this.name) ?? false;
+	}
+}
+
+/**
+ * Create a new {@link Context}.
+ *
+ * @param name - the unique name to give this context.
+ * @param defaultValue - the value used when the context is not present on a node.
+ * @returns the new context
+ */
+export function createContext<T>(name: string, defaultValue?: T): Context<T> {
+	return new Context(name, defaultValue);
+}
+
+export class MissingContextError extends Error {
+	constructor(name: string) {
+		super(`no value set for context "${name}"`);
+		this.name = 'MissingContextError';
+	}
+}
diff --git a/packages/ui/src/core/node.ts b/packages/ui/src/core/node.ts
new file mode 100644
index 0000000..2049b94
--- /dev/null
+++ b/packages/ui/src/core/node.ts
@@ -0,0 +1,11 @@
+export class Node {
+	readonly parent?: Node;
+	readonly children: Set<Node>;
+
+	contexts?: Map<string, unknown>;
+
+	constructor(parent?: Node) {
+		this.parent = parent;
+		this.children = new Set();
+	}
+}