Spacecraft measurements have revealed a hot, dense, CO2 atmosphere on Venus and a cold, thin, CO2 atmosphere on Mars. The surface pressure on Mars varies markedly with topographic elevation and probably with latitude and season. The lowest regions of the atmosphere of Venus have yet to be probed. Several thin micro wave-absorbing cloud decks, perhaps consisting of water droplets or compounds of mercury, exist in the atmosphere of Venus. It may be possible to explain the ionospheres of Mars and Venus in terms of the photochemistry of CO2 and lighter trace elements plus the effects of the impinging solar wind.