Throughout history, humans have transported and released plants and animals from one ecosystem to another for various reasons. Plants or animals moved into areas outside their historic or natural geographic range are considered nonindigenous species. This includes species transferred into the United States from foreign countries as well as those species moved from one region or watershed to another within the United States. In the past, nonindigenous species have been referred to as “exotics,” “transplants,” “nonnatives,” or “introduced species” (Shafland and Lewis 1984); in foreign countries, they are often called “alien” species. Within the United States alone, humans have intentionally or unintentionally introduced over 4,500 foreign species (U.S. Congress, Office of Technology Assessment (OTA) 1993). Although many terrestrial introductions are viewed as providing economic and social benefits to humans, all but a few intentional aquatic introductions have proven to be mixed blessings (U.S. Congress, OTA 1993; Steirer 1992; Courtenay and Williams 1992). This is not surprising, because considerable effort has been made to document possible beneficial aspects, whereas comparably little has been done to assess harmful impacts, especially over the entire lifespan of the introduced species. No unintentional aquatic introductions have been considered beneficial (Steirer 1992). In general, the environmental consequences of these introductions have been harmful; in some cases, they have been catastrophic (Taylor et al. 1984; U.S. Congress, OTA 1993).