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Two factors in reptile sex determination have been studied: (1) the presence or absence of heteromorphic sex chromosomes, and (2) the influence of temperature. Recognizable sex chromosomes are common in snakes and lizards, but are apparently rare in turtles and absent in crocodilians and the tuatara. Temperature-dependent sex determination (TSD) is common in turtles and has been reported in two lizards and alligators; however, data on TSD are available for few non-turtle species. Present findings on TSD suggest that (1) temperature actually determines sex rather than simply causing differential mortality, and (2) temperature controls sex determination in nature as well as in the laboratory. Only one study, however, has convincingly demonstrated the latter. Sex determination by nest temperature is proposed to interfere with the evolution of sex chromosomes and live-bearing (ovoviviparity); a negative correlation should thus be observed between TSD and sex chromosomes/live-bearing. Present evidence is consistent with these predictions. Possible selective advantages and disadvantages of the different sex-determined mechanisms are discussed, and an attempt is made to deduce their ancestries.