Mites (subclass Acari) are an extremely diverse group of arachnids, closely related to spiders and scorpions. Mites are small and ubiquitous -- even if your room looks perfectly clean, it is home to tens of thousands of tiny dust mites. Mites are among the most diverse subclasses of life, with over 45,000 known species, and an estimated total approaching one million. Because most mite species are microscopic and tropical, their diversity has been poorly characterized.

Although mites are the most successful group of arachnids, most of them are less than a millimeter in length, meaning we never see them. Dust mites are among the smallest mites, about a third of a millimeter in length. Immature mites may be smaller than a fifth of a millimeter. Some of the largest mites are ticks, the bloodsuckers that spread Lyme disease. Pliny the Elder called ticks are "the foulest and nastiest creatures that be."

Like some other arachnids, mites are among the oldest known terrestrial creatures, with fossils going back to the Devonian period, 400 million years ago. These mites lived among some of the earliest land plants. Like other common invertebrates, such as nematodes (transparent microscopic arthropod worms), mites are totally ubiquitous, having colonized pretty much every known terrestrial, freshwater, and marine habitat, including polar and alpine extremes. Mites are one of the few animals found in Antarctica. The three main lineages of mites are called Opilioacariformes, Acariformes and Parasitiformes.

In soils, mites can be found buried as deeply as 10 m (33 ft), in water almost freezing or as hot as 50 °C (122 °F), in barren deserts, deep sea trenches, etc. A typical square meter of forest floor litter may contain around one million mites representing 200 species in at least 50 families. Individual and diversity counts for mites outnumber practically any other animal except for nematodes.

To get rid of dust mites from your clothing, wash them at a high temperature. Dust mite excreta can cause various allergic conditions, such as hay fever, asthma and eczema and atopic dermatitis. To control mite population in the home, it is recommended that you frequently wash your blankets at high temperature.