Mars is a cold desert planet with a thin, dry carbon-dioxide atmosphere. The geology of Mars, however, shows signs of an active past, with hot-spot volcanism, and tantalizing signs of ancient water flows. While a cold, dead desert planet today, Mars' past may have been warmer and wetter, with liquid water during the first third of its history. This lecture reviews the properties of Mars, and describes the evidence for its active past. Recorded 2007 Nov 9 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.