Springs include a great variety of habitats, because many possible geological and ecological conditions intersect in any given spring.Available information on the arthropod fauna shows that springs contain a limited number of species of diverse origins, including groundwater, stream, and water-film inhabitants. There is a substantial number of spring-specialist species, many of them in distinctive genera, reflecting many independent invasions of spring habitats by various groups and subgroups of aquatic arthropods. Most of this diversity is present in cold water springs, though smaller numbers of distinctive elements occur in hot or in saline springs. The specialists of coldwater springs tend to show adaptations such as cold stenothermy and limited dispersal, but different species possess different suites of adaptations to the habitat, reflecting their evolutionary history and biology.Faunal differences among springs result from geographical differences (many species, though not as many genera, differ between eastern and western Canada), but within a given region reflect the variety of habitats and microhabitats that exists. Such variety means that except in very broad terms it is not possible to establish workable “definitions” for the range of spring types. Rather, we recommend that biologists adopt a few key descriptors, based on source geometry, water supply, temperature, chemistry, and persistence, to provide useful information about the sites in which they collect. The term “spring” should be used conservatively, to apply only to the area immediately around the point of groundwater issue, because conditions change rapidly farther away from this point.Some needs for the inventory (and protection) of springs and for more extensive sampling are summarized. Further taxonomic studies are required in several characteristic groups. Ecological work on the specialized species confined to springs is likely to be especially instructive.