How does synchronization work in Java?

synchronized ensures only one thread executes critical section at a time. Synchronized methods lock on 'this' (instance) or class object (static). Synchronized blocks lock on specified object, offering finer control. Each object has intrinsic lock (monitor). Thread acquires lock on entry, releases on exit. wait() releases lock and waits for notify(); notify()/notifyAll() wake waiting threads. Alternatives: ReentrantLock (explicit lock/unlock, tryLock, fairness), ReadWriteLock (multiple readers OR single writer). Synchronization prevents race conditions but can cause deadlocks.