Publisher Summary This chapter focuses on the general biology, classification, and ecology of the members of order Decapoda found in freshwaters of North America. Decapoda is the most diverse order of the class Malacostraca in marine and freshwater ecosystems. In the United States alone, there are 369 crayfish species, 45 shrimp species, and 6 crab species. All are characterized by terminal claws on the first three of five pairs of thoracic appendages and a branchial chamber enclosed within the carapace. Decapods are the largest motile invertebrates in North American freshwaters. Their size, activity, and omnivorous diet make them key players in the benthos of many ecosystems. For example, they can influence species distribution of snails within a community by selectively preying on certain size classes or shell morphologies, and they can be voracious herbivores that devastate vascular plants in ponds. Decapods, unlike practically all other North American invertebrates, are increasingly consumed by humans, especially in the southern United States. The chapter describes the morphology, physiology, phylogeny, evolution, and taxonomy of Decapods.