The paper tries to explain the introduction of state religions. I start by analyzing the meaning of state religion and the instruments of state intervention in religion. I then discuss the rulers' religious objectives and derive ten testable hypotheses about the factors favoring the introduction of state religions and about the choice of instruments. In the main section of the paper I examine the circumstances in which state religions have been introduced. The sample contains thirty-one state religions. The period stretches (...) from 2630 BC to 1871 CE. Eight hypotheses turn out to be consistent with the historical evidence. The frequencies are significantly higher than 50% in a binomial test. (shrink)