The Hydroptilidae are a large family of caddisflies (Trichoptera) with a worldwide distribution. They are commonly known as microcaddisflies or purse-case caddisflies, in reference to two characteristic traits of this family: Hydroptilidae are much smaller than other caddisflies, rarely exceeding 5 mm (0.20 in) in length. Their larvae do not build a protective case until the final instar of their growth. At that time however, they build a typically Purse-shaped case, either portable or stuck to the substrate, in which the larva finishes growth and pupates.

The caddisflies, or order Trichoptera, are a group of insects with aquatic larvae and terrestrial adults. There are approximately 14,500 described species, most of which can be divided into the suborders Integripalpia and Annulipalpia on the basis of the adult mouthparts. Integripalpian larvae construct a portable casing to protect themselves as they move around looking for food, while annulipalpian larvae make themselves a fixed retreat in which they remain, waiting for food to come to them. The affinities of the small third suborder Spicipalpia are unclear, and molecular analysis suggests it may not be monophyletic. Also called sedge-flies or rail-flies, the adults are small moth-like insects with two pairs of hairy membranous wings. They are closely related to the Lepidoptera which have scales on their wings; the two orders together form the superorder Amphiesmenoptera.