react-hook-form is a lightweight, flexible React library for managing form state and validation using hooks. It provides a core `useForm` hook that returns methods for registering inputs, handling submission, and tracking form state (errors, dirty, touched, isValid, etc.). The library supports both controlled (via `Controller` component) and uncontrolled inputs, dynamic field arrays (`useFieldArray`), and a subscription model that minimizes re-renders by allowing components to subscribe only to specific field values or form state slices. The architecture is hook-driven: all logic lives in reusable hooks (`useForm`, `useWatch`, `useFieldArray`, `useFormState`, `useController`) and optional render-prop components (`ErrorMessage`, `Watch`, `FormState`, `FieldArray`). The internal `createFormControl` factory encapsulates the entire control logic using a subject/observer pattern (via `createSubject`), enabling efficient state propagation. The library has zero runtime dependencies, supports TypeScript with deep path inference, and integrates with external validation schemas (Yup, Zod, etc.) through a resolver interface. Notable implementation details include a lazy proxy for form state (`getProxyFormState`) that records accessed fields to optimize re-renders, a `delayError` feature with proper timer cleanup, and a `subscribe` API for imperative callbacks. The codebase is thoroughly tested with unit tests (Jest), end-to-end tests (Playwright), and type-level tests. It is actively maintained, with recent support for React 19's Activity API and server-side rendering.