Observations from space vehicles have vastly altered our conceptions of Mars and Venus. Mars has changed from an earth‐like planet with water ice caps into a heavily cratered, coreless object whose polar caps are dry ice and whose tenuous atmosphere is CO2. Venus has been found to be blanketed by clouds and a dense CO2atmosphere that has driven the surface temperature to 700–800°K. Attention is drawn to the practical returns we can expect from comparative studies of these three planets: for understanding of the origins of life, the potential development of a runaway greenhouse effect on earth, and atmospheric circula