Perhaps 10,000–20,000 species of freshwater invertebrates are extinct or endangered as a result of human impacts such as damming and other habitat destruction, pollution, overharvest, introductions of nonnative species, and climate change. Endangered invertebrates are not spread evenly across the Earth, but are concentrated in “hot spots” such as ancient rivers, lakes, and aquifers. Species with small ranges and poor powers of dispersal that live in heavily developed areas or semiarid regions are especially at risk. Existing legal protections for freshwater invertebrates often are inadequate, and will need to be substantially improved if widespread extinctions are to be avoided.