It is often stated that Mars and Earth had similar environmental conditions early in their history and that life might therefore have originated on Mars as well as on Earth. However, the atmospheric conditions required to produce and sustain a warm, wet climate on early Mars remain uncertain. State-of-the-art greenhouse models predict global mean surface temperatures early in Mars' history that differ little from today's, unless special conditions are invoked. The greatest difficulty the models have is coping with a faint early Sun. ¿ 1998 American Geophysical Union