Recent work by Forget and Pierrehumbert (Science 278, 1273, 1997) has shown that CO₂ clouds can, in some instances, exert a strong warming effect on planetary climates. These authors suggest that CO₂ clouds were responsible for a warm (Earth-like?) climate on Mars during the early part of the planet's history. How robust is this conclusion, though, and could other factors (e.g., additional greenhouse gases such as CH₄) be important? Also, how warm would the Martian climate need to have been to create the surface features that we see today? These questions will be discussed, along with their implications for the width of the habitable zone around the Sun and the possibility of finding Earth-like planets around other stars.