Water vapor plays a vital role in shaping weather and climate on Earth. Hence, monitoring water vapor is critical if we are to explain and predict the behavior of the climate system. Unfortunately, measuring and analyzing water vapor on the time and space scales needed for this purpose have proven elusive. Therefore, it is appropriate and timely for the international climate research community, through the Global Energy and Water Cycle Experiment (GEWEX), to focus a project around water vapor. To this end, a GEWEX Global Water Vapor Project (GVaP) has been proposed, and draft Science and Implementation Plans have been developed. As requested by the U.S. Global Change Research Program (USGCRP), the National Research Council’s (NRC) GEWEX Panel has reviewed these plans with an eye toward U.S. priorities.