Mites belonging to several unrelated groups are commonly found in freshwater habitats. The true water mites comprise a taxon of actinedid acariform mites, which is referred to as Hydrachnidiae. Hydrachnidiae have flourished to become by far the most numerous, diverse, and ecologically important group of freshwater arachnids. Various families in certain suborders of Acariformes (Actinedida, Oribatida, and Acaridida) and Parasitiformes (Gamasida) have independently invaded freshwater habitats and become adapted for living there. Compared to Hydrachnidiae, these groups are less diverse taxonomically, usually less abundant, and relatively conservative in morphology and habits. There are no truly aquatic spiders in North America, but many species of nurseryweb spiders (family Pisauridae) are associated with aquatic habitats. This chapter describes the anatomy, morphology, physiology, reproduction, life history, phylogeny, evolution, ecology, and taxonomy of water hydrachnidiae and other arachnids, with focus on those found in freshwaters of North America.