Vernal pools, broadly defined as ephemeral wetlands that predictably form in permanent basins during the cooler part of the year but which dry during the summer months, are distributed throughout the world. In the U. S., they are particularly abundant on the Pacific Coast and in various forms in the glaciated landscapes of the north and northeast. Vernal pools are ecosystems that have evolved in a balance between isolation and connectedness. Because of isolation at several scales, the vernal pools biota includes many regionally endemic species. Because of connectedness, vernal pools also share many taxa with continent-spanning distributions at the generic and species level. Vernal pools serve an important local biodiversity function because of their connection to surrounding terrestrial habitats. Along with other ephemeral wetlands, they are the primary habitat for animal species that require relatively predator-free pools for feeding or breeding, including many amphibians. The recent U. S. Supreme Court decision (SWANCC), which deemed “isolated” wetlands to be outside the class of “waters of the United States,” places some significant but unknown proportion of vernal pools at risk. In the worst case, the consequences could be immediate reductions in biodiversity at a local level, and regional reductions over longer periods of time. Ideally, federal law should be rewritten to establish unambiguously the value of ephemeral wetlands. It will also be necessary for conservationists to educate the public and to bring the issue of vernal pool protection to the notice of their local and state governments.