Several episodes of Trek have been censored or simply not shown over the years. Assuming your question relates to the UK, then four instances are notable;
TOS : Empath, Whom the Gods Destroy, Plato's Stepchildren and Miri were all removed from the broadcast schedule for over 10 years due to containing subjects of "madness, torture, sadism and disease" in what was considered by the BBC to be a show intended for children.
