Epilobium is a genus in the family Onagraceae, containing about 160-200 species of flowering plants with a worldwide distribution. They are generally abundant in the subarctic, temperate and subantarctic regions, whereas in the subtropics and tropics they are restricted to the cool montane biomes, such as the New Guinea Highlands where they are plentiful.

The taxonomy of the genus has varied between different botanists, but the modern trend is to include the previously recognised genera Boisduvalia, Chamaenerion, Chamerion, Pyrogennema and Zauschneria within Epilobium. Chamerion might be distinct, however, according to Peter H. Raven, who has extensively studied the willowherbs and merges the other segregate genera into Epilobium. Fringed willowherb (E. ciliatum) is likely a cryptic species complex; apparently these plants also commonly hybridize with their congeners.

Most species are known by the common name willowherbs; in particular, those that were once separated in Boisduvalia are called spike-primroses or boisduvalias. Yet other species, namely those in the Chamerion group, are also known as fireweeds.

They are mostly herbaceous plants, either annual or perennial; a few are subshrubs. The leaves are mostly opposite or whorled, alternate in a few species, simple, and ovate to lanceolate in shape. The flowers have four petals often notched. These are usually smallish and pink in most species, but red, orange or yellow in a few, and large and bright magenta in the Chamerion group. The fruit is a slender cylindrical capsule containing numerous seeds embedded in fine, soft silky fluff which disperses the seeds very effectively in the wind.

Willowherbs are typically very quick to carpet large swathes of ground and may become key or dominant species of local ecosystems. In and around the United Kingdom, for example, rosebay willowherb (E. angustifolium) is widely found in mesotrophic grassland dominated by false oat-grass (Arrhenatherum elatius), cock's-foot (Dactylis glomerata), and red fescue (Festuca rubra), while great willowherb (Epilobium hirsutum) is found in mesotrophic grassland with stinging nettle (Urtica dioica). These two willowherb species are also seen to dominate open habitat early in ecological succession, to the virtual exclusion of other plant life. Broad-leaved willowherb (Epilobium montanum) is found characteristically, though not abundantly, in the mesotrophic grasslands with meadowsweet (Filipendula ulmaria) and sometimes the uncommon Greek valerian (Polemonium caeruleum), which are peculiar to the Pennines. Most willowherbs will not tolerate shade trees and thus are limited to more recently disturbed patches, yielding to other plants over time. Consequently, though the genus contains many pioneer plants, rather few of them are invasive weeds of major importance.