The Opilones are closely related to spiders, but their body does not have the distinct division between the cephalothorax and abdomen seen in the spiders. Nor do they have poison glands or silk glands, so they cannot spin webs. Their eyes are often located in a turret-like structure (see Leiobunum flavum below and Plangium sp. at the bottom of the page). Many are omnivorous, some are carnivorous, feeding mainly on small arthropods and snails, but will eat anything they can get hold of. All have eight long legs, though often one or more can be missing. There are 5 000 species world wide.

Little is known of their mating habits. There are reports that some species may have "courting grounds" where females gather to choose a mate. At these places the males fight using their chelicerae (fangs) to bite off the legs of their rivals. In other species, the female has less choice. When the male finds a suitable female he chases her and tangles his legs with her preventing her escape. Unlike spiders male opilones have a penis.

Females have an long ovipositor, and normally it is located in a sheath, but when the time comes to lay eggs it is extended (see Leiobunum rotundum on the left). The ovipositor is shaped like a drill bit with chitinous teeth for digging. Oviposition takes place soon after mating. In some species the male guards the female while she oviposits, stroking her body gently with his legs. The eggs are laid in damp soil, moss, rotting wood, etc. shortly after mating. The female will lay several batches of eggs in different locations. In all she can lay hundreds of eggs.

Life span in temperate regions is usually just one year.

Parthenogenesis has been reported in some species.