The name of this order, which means "hairy winged", suggests their close relationship with the Lepidoptera (moths and butterflies). The Trichoptera are one of the most diverse and conspicuous aquatic macroinvertebrate orders. Caddisflies are found in most aquatic habitats, and many have specific habitat and water quality requirements, so they are good indicators of habitat condition. Caddisfly families are adapted to a diversity of habitats, and this is reflected in the wide variety of cases they construct.

The shelters that most caddisflies build are probably the group's most remarkable feature. These cases may be stationary or portable, and are used for protection, feeding, and moving water past the gills. The shape and material of the cases often reflects the type of habitat in which they live - caddisfly bases may be built from sand grains or plant pieces, and may be tubular, square, or even coiled. One especially unusual genus (in the Helicopsychidae) was initially identified as a snail by the scientist who discovered it! The cases may also be quite colorful; one of the most famous Northwest caddisflies is the large black and orange October or fall caddisfly. Some common and successful caddisflies build a stationary shelter and construct complex silken nets to capture drifting organic debris and tiny invertebrates.

Caddisflies are very important in stream food chains; they eat both plant and animal material. Because caddisflies undergo complete metamorphosis, they are confined to the water throughout most of their life cycle, and thus may be eaten by fish as larvae, pupae and adults. Caddisflies are well-known to fishermen for just that reason.

Many caddisfly taxa are fairly easy to identify to the family level, and even to genus. Others, particularly the case maker families, will take some time to learn and as with most aquatic invertebrates, can be especially difficult to identify as early instars. In general, the more straightforward families are separated out first in this section's family key.

In the wetlands of the Pacific Northwest, you are most likely to find the caddisfly families Hydroptilidae (micro caddisflies or purse makers), Leptoceridae (long-horned case makers) and Limnephilidae (northern case makers).