The planets Venus, Earth, and Mars in our solar system demonstrate the major roles played by thermal history, geophysical processes, geological activity, and interactions with the interplanetary environment in determining atmospheric properties, climate, and the history and accessibility of liquid water at and near the planets'surfaces. The three planets provide different "boundary conditions" on planetary evolution, with the result being that they have very different water histories. We are just now beginning to understand the complex interplay that can occur between the various components of a planet and how they can affect the resulting climate and history of water. Given the importance of water as a lagging indicator of the history of the planet and as a leading indicator of the potential for life, it is premature to claim that we understand what determines the potential that a planet has for life. The discovery of planets around other stars brings these issues to the fore, in that they affect our ability to determine the implications of the existence of extrasolar giant planets (which have been discovered) for the existence of extrasolar terrestrial planets (which have not) and for the potential that such planets would have for the occurrence of life.