Java multithreading enables parallel execution for improved application performance. Thread lifecycle includes new, runnable, blocked, waiting, and terminated states. Implement Runnable or extend Thread for creating threads. ExecutorService manages thread pools efficiently instead of creating threads manually. synchronized keyword ensures mutual exclusion on shared resources. volatile guarantees visibility of changes across threads. Atomic classes provide lock-free thread-safe operations on single variables. Concurrent collections like ConcurrentHashMap offer thread-safe data structures. Locks from java.util.concurrent.locks more flexible synchronization than synchronized. ReentrantLock supports fairness policies and tryLock. ReadWriteLock allows concurrent reads with exclusive writes. CountDownLatch coordinates thread completion. CyclicBarrier synchronizes threads at a common point. Semaphore controls access to limited resources. CompletableFuture enables asynchronous programming with composable futures. Fork/Join framework parallelizes recursive tasks. Avoid deadlock by consistent lock ordering. Use ThreadLocal for per-thread state. Thread pools should have bounded queues to prevent resource exhaustion. Java Memory Model defines thread interaction through shared memory.