Present knowledge of the occurrence, properties, and composition of tektites is reviewed; and the principal theories which have been advanced to explain their origin are critically discussed. Nininger's lunar impactite theory, modified in accordance with the suggestions of Varsavsky is most free from objection, but if Kopal's contention that tektites must have had an origin closer to the Earth is well founded, Hardcastle's plastic sweeping theory seems likely to repay closer attention than it has so far received.