The neutral K meson or neutral kaon, K0, and its antiparticle, K(bar)0, form a remarkable quantum-mechanical two-state system that has played an important role in the history of elementary particle physics. Indeed, ever since the discovery of K0 half a century ago, neutral kaons have been a rich source of unique and facinating phenomena associated with their production, decay and propagation in both vacuum and matter. This overview conveys the unique beauty of a quantum-mechanical system that contains so many of the aspects of modern physics.