This chapter discusses the very important class of states referred to entangled quantum states. Detailed discussion of polarization entangled statePolarization entangled state, Bell statesBell states, spatial mode entangled statesEntangled states, hybrid entangled statesEntangled states, hyperentangled states, and states with higher dimensional entanglementHigher dimensional entanglement is presented. The very important measure of CHSH (Clauser, Horne, Shinmoy, Holt) inequality is discussed both for entangled photons and distinguishable photons. A brief discussion of GHZGHZ state (Greenberger, Horne, Zeilinger) state is also given.