This study examines when mandatory clerical celibacy was instituted in the Czech lands. At first it was only demanded of candidates to become bishops while other priests regularly had wives and children up to the 12th century. The Papal Curia first intervened in favour of celibacy in 1143 through a mission by Cardinal Guido when married clerics were removed from their posts. Another came with Cardinal Peter in 1197 when he (unsuccessfully) demanded that those being ordained also take a vow of purity. Celibacy was then enforced after the Fourth Council of the Lateran in 1215.