The complete body of knowledge of the lepton spectrum as it is known today is presented and critically examined. Whilst the emphasis is primarily on the experimental determination of leptonic properties, the authors also summarise the many theoretical endeavours that have enriched and guided the field, and continue so to do. Complete accounts are given of the current situation on neutrino mass determination, on neutrino oscillations and on the properties of the tau and search for its associated neutrino. The subject of lepton conservation is discussed and all the experimental evidence for non-conservation collected together. The search for leptons other than those belonging to the three familiar generations is reviewed. Finally, a brief account is given for the limits on leptonic properties imposed by cosmological arguments.