Mars, fourth planet from the Sun, 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 will review the properties of Mars, and discuss the evidences of its active past. Recorded 2006 Nov 13 in 100 Stillman Hall on the Columbus campus of The Ohio State University.