Decoy state quantum key distribution (QKD) has been proposed as a novel approach to improve dramatically both the security and the performance of practical QKD set-ups. We report the theory of the subject. Moreover, we report the world’s first experimental demonstrations of decoy state QKD over 15km and 60km of commercial Telecom fibers, done by our group at the University of Toronto. Decoy state QKD is ready for immediate commercial applications.