Epilobiums are mostly herbaceous plants, either annual or perennial; a few are subshrubs. The leaves are opposite or rarely whorled,^[4]: 354 simple and ovate to lanceolate in shape. The flowers are actinomorphic (radially symmetrical) with four petals that may be notched.^[4][5] These are usually smallish and pink in most species, but red, orange or yellow in a few. 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.

The genus name derives from the Greek words "epi" meaning "upon" and "lobos" meaning "lobe", with reference to position of the petals above the ovary.^[6]

A number of Epilobium species with slightly asymmetrical (zygomorphic) magenta flowers and alternate leaves are placed in a Chamaenerion group and are further separated by some authorities into a genus Chamerion. However, Chamerion has not been universally accepted.^[7][8][9]

Willowherbs sensu lato are typically very quick to carpet large swathes of ground and may become key or dominant species of local ecosystems. In the United Kingdom, for example, rosebay willowherb (Chamaenerion angustifolium) is widely found on mesotrophic soils dominated by false oat-grass (Arrhenatherum elatius), cock's-foot grass (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 also 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). 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.

Epilobium species are used as food plants by the caterpillars of certain Lepidoptera species, including:
Geometridae

• Grey pug (Eupithecia subfuscata), recorded on several species

Noctuidae

• Mouse moth (Amphipyra tragopoginis), recorded on several species
• Small angle shades (Euplexia lucipara), recorded on several species
• Australian grapevine moth (Phalaenoides glycinae), recorded on fringed willowherb (E. ciliatum)

Sphingidae

• Elephant hawk-moth (Deilephila elpenor), recorded on several species
• Small elephant hawk-moth (Deilephila porcellus), recorded on several species
• White-lined sphinx (Hyles lineata), recorded on several species

The main use of Epilobium by humans is as a herbal supplement in the treatment of prostate, bladder (incontinence) and hormone disorders. Many of the small willowherb species are nuisance weeds in gardens. Though few are regularly used as ornamental plants, the larger willowherbs may be attractive in ruderal locales. One of the most frequently recognized members of the genus is the circumboreal fireweed (E. angustifolium), known as rosebay willowherb in the United Kingdom. It rapidly colonizes burnt ground; during the bombing of London in World War II many of the derelict bomb sites were soon covered with these plants, bringing a splash of colour to what was otherwise a very grim scene. It is the floral emblem of Yukon in Canada, Hedmark in Norway and Southern Ostrobothnia in Finland.

Fireweed is used as a sweetener in north-western North America. It is put in candy, jellies, ice cream, syrup, and sxusem ("Indian ice cream"). In the late summer its flowers yield pollen and copious nectar which give a rich spicy honey. Its young leaves, roots, and shoots are edible (if somewhat bitter), and rich in provitamin A and vitamin C. The Dena’ina found them also useful as food supplement for dogs and applied sap from the stem to wounds, believing it to have antiinflammatory properties.

Several researchers have studied this taxon. Heinrich Carl Haussknecht in the late 19th century and Peter H. Raven about a century later researched the phylogeny, systematics, and taxonomy of willowherbs. Peter Michaelis' studies of this genus paved the way for understanding of extranuclear inheritance in plants.

The following species are recognised in the genus Epilobium:^[10]

Formerly placed here edit

• Stylidium tenellum (as E. tonkinense)

It is possible to distinguish between leaves of different Epilobium species using high-accuracy FT-IR method based on attenuated total reflection (ATR) without time-consuming preparation.^[11]

• Bleeker, Walter; Schmitz, Ulf & Ristow, Michael (2007): Interspecific hybridisation between alien and native plant species in Germany and its consequences for native biodiversity. Biological Conservation 137 (2): 248-253. doi:10.1016/j.biocon.2007.02.004 (HTML abstract, appendix reserved for subscribers)
• Steenkamp, V; Gouws, M.C; Gulumian, M; Elgorashi, E.E. & van Stade, J. (2006): Studies on antibacterial, anti-inflammatory and antioxidant activity of herbal remedies used in the treatment of benign prostatic hyperplasia and prostatitis. Journal of Ethnopharmacology 103 (1): 71–75. doi:10.1016/j.jep.2005.07.007 PDF fulltext
• Wagner, Warren L.; Hoch, Peter C. & Raven, Peter H. (2007). Revised classification of the Onagraceae (PDF). Systematic botany monographs. Ann Arbor, Mich: American Society of Plant Taxonomists. ISBN 978-0-912861-83-8. Retrieved 2017-02-22.
• "Onagraceae - The Evening Primrose Family: Chamaenerion". Smithsonian Museum of Natural History. Retrieved 2017-02-22.
• Wagner, W.L. & Hoch, P.C. [2009b]: Evening Primrose Family website – Epilobium. Retrieved 2009-JAN-26.