At a time when most astronomers believed that GRBs were galactic (see however an earlier suggestion by Bohdan Paczynski ) with Eichler, Livio and Schramm, Piran proposed that GRBs originate from cosmological neutron star binary mergers, a model that is generally accepted today. During the early nineties when the cosmological vs. galactic debate took place, Piran was one of the strongest and most vocal proponents of cosmological origin, which was confirmed in 1997 with the discovery of cosmological redshifts from GRB's afterglow. Even before the cosmological origin of GRBs was discovered Piran laid the foundation to the generally accepted cosmic fireball model. He suggested that GRBs herald the formation of a newborn black hole. Later on, together with Re'em Sari and other collaborators, Piran further developed the theory of GRB afterglows, in a paper which has by now more than 1000 citations, and of GRB jets. His review papers are the standard literature on this subject.