Venus is the “exoplanet next door,” a key to understanding whether “Earth-sized” means “Earth-like” elsewhere in the Universe. Recent evidence of possibly current volcanism, and the suggestion of nascent plate tectonics on its surface, have raised questions about the geological evolution of its surface that wait to be answered by future missions. The runaway greenhouse effect in Venus atmosphere provides insights into the process of global warming on Earth. The heavily shrouded surface blocks use of orbital cameras in all but a few windows near 1 micron, so our knowledge of surface mineralogy remains limited. Finally, controversy remains as to the fate of Venus’ water, much of which may remain locked in its interior. This talk presents Venus as a cornerstone of comparative planetology, key to understanding geological processes.