Terrestrial planet climate studies provide a platform for the discussion of the climate changes currently affecting the Earth. The apparent stability of the Earth’s climate over the last few thousand years is deceptive, since it was preceded by huge changes taking place in the past, sometimes (during the most recent terrestrial ice ages, for example) on very short time scales of centuries or even decades. We now know that dramatic changes in climate, and potential habitability, have also taken place on Mars, and probably on Venus and Titan as well, although the time scales there remain uncertain. Amelioration of the current problem on Earth requires a very complete understanding of the processes involved. Applying models of such processes to Venus and Mars not only allows us to develop detailed scenarios and possible histories for those extreme variants of the terrestrial situation, it shows up deficiencies in our understanding that could make important differences to climate forecast for the near-future of life on Earth.