React Hooks revolutionized state management and side effects in functional components. useState provides local state management with array destructuring. The setter function can accept a value or updater function. useEffect handles side effects: data fetching, subscriptions, DOM manipulation. The dependency array controls when effects re-run. Empty dependencies mean run once on mount. useCallback memoizes functions to prevent unnecessary re-renders. useMemo memoizes computed values for performance optimization. useRef maintains mutable references without causing re-renders. useContext provides access to React context without prop drilling. useReducer manages complex state logic similar to Redux. useImperativeHandle customizes ref exposure to parent components. Custom hooks encapsulate reusable logic. Rules of Hooks: call hooks at top level, only in React functions. The ESLint plugin enforces these rules automatically. Hooks enable better code organization compared to lifecycle . Performance optimization requires understanding when re-renders occur. useLayoutEffect runs synchronously after DOM mutations. React 18's useDeferredValue and useTransition improve user experience. Mastering hooks is essential for modern React development.