The genus Macrobrachium, or river shrimps, is represented in the United States by 6 species: M. acanthurus, M. carcinus, M. faustinum, M. heterochirus, M. ohione, and M. olfersii. River shrimps are the largest fresh-water crustaceans in North America, and specimens of the largest species, M. carcinus, can be the size of lobsters. Most of these species are distributed among the southern states of the Gulf Coastal Plain, Mexico, and the Caribbean Islands. Macrobrachium ohione is known also from the Red River drainage in Arkansas and Oklahoma, and the Mississippi River-Ohio River drainage in Illinois, Indiana, and Ohio. Because of their complex amphidromous life cycle, populations of Macrobrachium are vulnerable to anthropogenic disturbances such as construction of impoundments, water pollution, commercial fishing, and introductions of exotic species. Preliminary data from Mississippi and Texas support the notion that populations of river shrimps are restricted by impoundments, and populations, in general, are in decline. A review of the species and a revised key are presented.