Entanglement, the greatest mystery in quantum mechanics, describes the instantaneous link between distant particles, challenging Einstein's local realism. The probabilistic theory, founded on superposition and wave-particle duality, faced opposition from Einstein. However, Bell's theorem created testable inequalities. Experiments by Clauser, Aspect, and Zeilinger consistently confirmed non-locality, proving entanglement is real and paving the way for advancements like quantum computing and teleportation.