Mites (Acari) are the only arachnids to have diversified within freshwater habitats. There have been several independent invasions of freshwater by mites from different taxa, but the most successful group is the ‘true water mites,’ Hydrachnida, from the prostigmatan cohort Parasitengona. There are more than 6000 described species of water mites found in all types of water bodies on all continents except Antarctica. Water mites have a complex life cycle that includes a larval stage, parasitic on insects, and predatory nymphal and adult stages. All major groups of aquatic insects (with the exceptions of Ephemeroptera and Megaloptera) are hosts of larval water mites. Parasitism by mites can reduce lifespan and fecundity of hosts. The predatory stages of water mites feed mainly on small crustaceans, other water mites, and the eggs, larvae, and pupae of aquatic insects. In simple communities, water mites can act as top predators. Although other invertebrates and fish occasionally eat water mites, it is more usual that they avoid consuming them, possibly due to distasteful secretions from skin glands. Water mites frequently achieve high densities and/or diversities, and their assemblages are good indicators of organic pollution; however, they have so far been underemployed in freshwater monitoring programs.