Annual or perennial herbs, or shrubs, rarely trees to 30 m tall, often with epidermal oil cells, usually with internal phloem. Leaves simple, spirally arranged, opposite, or occasionally whorled, entire or toothed to pinnatifid; stipules present and usually caducous, or absent. Flowers perfect and hermaphroditic or occasionally unisexual, actinomorphic or zygomorphic, (2-) 4(-7) -merous, axillary, in leafy spikes or racemes or solitary, or occasionally in panicles, all but Ludwigia with distinct floral tube, nectariferous within. Sepals green or colored, valvate. Petals as many as sepals or rarely absent, variously colored, imbricate or convolute and occasionally clawed. Stamens as many as sepals in one series or 2 × as many as sepals in 2 series [in Lopezia Cavanilles reduced to 2 or 1 plus 1 sterile staminode]; anthers versatile or basifixed, dithecal, sometimes cross-partitioned, opening by longitudinal slits; pollen grains almost always united by viscin threads, shed as monads, tetrads, or polyads. Ovary inferior, with as many carpels and locules as sepals, septa sometimes thin or absent at maturity; placentation axile or parietal, ovules 1 to many per locule, in 1 or several rows or clustered, anatropous, bitegmic; style 1; stigma with as many lobes as sepals or clavate to globose. Fruit a loculicidal capsule or indehiscent nut or berry. Seeds small, smooth or variously sculptured, sometimes with a coma [or wing], with straight oily embryo, endosperm lacking.

Seventeen genera and ca. 650 species: widespread in temperate and subtropical areas, but best represented in W North America; six genera (two introduced), 64 species (11 endemic, 11 introduced), and five natural hybrids (two endemic) in China.

Onagraceae are a well-defined, monophyletic family in the order Myrtales, with a sister relationship to Lythraceae. Within the order Myrtales, the Onagraceae are distinguished by a number of features including (1) a distinctive 4-nucleate embryo sac; (2) abundant raphides in vegetative cells; (3) paracrystalline beaded pollen ektexine; and (4) pollen with viscin threads.

Some species of Oenothera are grown for the oil in their seeds, which contains gamma linolenic acid (GLA), used for medicinal purposes. Several species of Onagraceae also are cultivated in China for their horticultural value, including species of Fuchsia Linnaeus (generally distinguished by having large, tubular, red or orange flowers and fleshy berries) and Clarkia Pursh (distinguished by having stigmas with commissural lobes with dry, unicellular papillae, and dry, elongate capsules similar to those of Epilobium but lacking comas on the seeds) . The most commonly cultivated Fuchsia is F. ×hybrida Hort. and the related F. magellanica Lamarck in F. sect. Quelusia (Vandelli) Candolle from South America; F. triphylla Linnaeus, in F. sect. Fuchsia, from Hispaniola, is known from only one gathering in Fujian. Similarly, Clarkia amoena (Lehmann) A. Nelson & J. F. Macbride is widely cultivated in China, whereas C. pulchella Pursh is known from only one gathering in Xizang; both species are native to W North America. There are no naturalized species of either Clarkia or Fuchsia in China.