In Orthodox Judaism, pronouncing it is considered sacrilege. As a result, when the Masorites were placing vowel points in the manuscripts of the Taanakh, they placed the vowel points for Adonai ("Lord" or "Master," a title of God), indicating that the reader should pronounce "Adonai" in its place. Thus, early translators into English took the vowel points at face value and rendered the tetragrammaton as "Jehovah". (The letters Y and J were not originally distinguishable.) Further research has since shown "Yaweh" to be a more probable pronunciation.
Most English translations of the Bible follow the Masoretic convention by printing the Tetragrammaton as "the LORD."