The worldwide transfer and introduction of nonindigenous species (NIS) by human activities is having significant and unwanted ecological, economic, and human-health impacts. Although marine invasions have received relatively little attention compared to terrestrial and freshwater communities, the extent and consequences of these invasions are becoming increasingly clear. For example, approximately 400 NIS are known to be established in coastal marine waters of the U.S. and over 200 of these can occur in a single estuary. Some of these species have become numerical dominant and significantly alter key population, community, and ecosystem-level processes. In short, it is now clear that invasions by NIS are fundamentally altering the structure and function of our coastal ecosystems.