The European green crab Carcinus maenas has invaded many places around the world. Where it arrives, it usually wreaks havoc with local shellfisheries and native ecosystems. Although marine invaders are increasingly common, even the worst case scenarios have been met with fatalistic acceptance by marine ecologists. When the green crab invaded the San Francisco Bay in 1992, we became worried about the potential impacts to our coast and began to devise strategies for its control (Lafferty and Kuris, Ecology 1996). In fact, the green crab has eaten our native fauna as voraciously as expected. It is also spreading remarkably fast. Already, it is in bays along the Washington coast. To understand how to battle the green crab, we first needed to understand why it was so successful here. We surveyed crabs around the world and found that introduced green crabs were larger than native green crabs due to increased growth and/or survivorship. This enhanced performance occurred because they suffer less from predators and parasites. Release from natural enemies may be the primary contributor to the success of invaders and supports the likelihood that classical biological control may be a feasible means to reduce the impacts of these introduced crabs. This requires adapting strategies from terrestrial pest control (where biological control is common) to the marine environment. Biological control, though potentially effective, can carry risks to the environment. One agent we feel has the potential to be safe is the European parasitic barnacle, Sacculina carcini. This barnacle is a parasitic castrator. If the barnacle could achieve sufficient rates of infection, it could substantially reduce the green crab population. Presently, we are culturing the barnacle so that we can learn if it is able to infect our native crabs. If not, it may prove worthy of further consideration as a control option.