The first documented case of mandatory clerical celibacy was the Council or Synod of Elvira, held in Spain sometime towards the beginning of the fourth century. From there it spread gradually east until celibacy was required for all western clerics. This practice never took hold in the East.
In Eastern Orthodoxy, bishops and monastics are celibate. However, candidates for the priesthood and diaconate are allowed to marry if they do so before ordination. A married priesthood is technically allowed in the Eastern Rite Catholic churches. However, in some jurisdictions, Latin Rite Catholic bishops have not recognized married Uniate priests as being valid, which has been a contributing factor to the return of several Eastern Rite groups to Orthodoxy.