We review experimental knowledge on what are usually called heavy leptons, but we call new leptons. These are leptons other than the electron, muon, and their associated neutrinos. The measured properties of the tau lepton are surveyed, including a discussion of the inconsistencies in present knowledge of its one-charged-particle decay modes. We summarize experimental limits on the existence of other possible leptons. We conclude the review with a discussion of searches which can be made with the new and proposed particle colliders.