The bark lice are a small order of minute, fungivorous, algivorous or detritivorous insects which can be found in most terrestrial habitats. They are most common on tree trunks, in foliage and on conifer needles, and many species have adapted to life in human dwellings. These include the well known booklice (Liposcelididae).

Bark lice have a soft body and a large head with long antennae, usually well developed and protruding compound eyes, a prominent forehead (frons), and chewing mouthparts. They often have two pairs of wings, but the wings may also be reduced or altogether absent. Sexual wing dimorphism is common, with the male usually being winged and the female wingless. In some species both sexes display wing dimorphism. Inquilinous species are usually wingless. Together with the true lice (Phthiraptera), the bark lice form a natural (monophyletic) group Psocodea. A unique feature of this group is the ability to take up moisture directly from the atmosphere, a trait which has enabled the members to occupy very dry habitats. The bark lice alone do not constitute a natural group, as the true lice appear to have evolved from the bark louse suborder Troctomorpha.