North America harbors about 390 native species of crayfishes, 75% of the world's total. In this article, we highlight the threats posed by nonindigenous crayfishes to freshwater ecosystem function, fisheries, and the biodiversity of native crayfishes; draw some lessons for North American freshwater conservation from the experience with nonindigenous crayfishes in Europe; and review existing regulations that address the introduction of nonindigenous crayfishes. Most North American crayfishes have naturally small ranges in the southeastern United States, rendering them very vulnerable to environmental change. In contrast, Europe has only five, broadly distributed, native crayfishes, all of which have been greatly affected by environmental changes, especially the introduction of nonindigenous crayfishes (mostly from North America). In response, many European governments have adopted strict regulations to protect native crayfishes. The loss of thousands of populations of native European crayfishes an...