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Tardigrades are hydrophilous micrometazoans with a bilaterally symmetrical body and four pairs of lobopodous legs usually terminating in claws (and/or digits in some marine species). Tardigrades have commonly been called "water bears" due to their bear-like appearance, legs with claws, and slow lumbering gait. The name "il Tardigrado" (slow-stepper) was introduced to describe the slow, tortoise-like movement of the animal. Thus far, more than 1000 species of tardigrades have been described in two classes (Heterotardigrada and Eutardigrada) from marine, freshwater, and terrestrial habitats (including caves), but the total number of existing species has not been predicted with species richness estimators developed in recent years for biodiversity studies. Generally convex on the dorsal side and flattened on the ventral side, the body is indistinctly divided into a head (cephalic) segment, three trunk segments each bearing a pair of legs, and a caudal segment with the fourth pair of legs directed towards posterior. Although tardigrades range in body length from 50 m in juveniles to 1200 m in adults (both excluding the last pair of legs), mature adults average 250-500 m with very few species exceeding 800 m. Although some species exhibit color in the gut, storage cells, epidermis, and/or cuticle, most are translucent or whitish (opaque). This chapter describes the anatomy, morphology, physiology, reproduction, life history, phylogeny, evolution, ecology, and taxonomy of tardigrades, with focus on those found in freshwaters of North America.