The amphipod crustacean family Gammaridae is represented in the freshwaters of North America by eight genera and 81 described species; numerous other species are still undescribed. These eight genera, with the number of described North American freshwater species in parentheses, include: Gammarus (9), Crangonyx (18), Synurella (4), Apocrangonyx (6), Stygonectes (29), Stygobromus (10), Bactrurus (3), and Allocrangonyx (2). Ecologically, the freshwater gammarids are an important group of aquatic invertebrates, with species found in a variety of biotopes, including lakes, streams, ponds, swamps, springs, and subterranean waters. In order to facilitate the identification of genera and the determination of species, analytical keys with accompanying illustrations are presented. Of further assistance are distributional maps showing the ranges of many of the species.