Herbs, shrubs, or trees; young stems often quadrangular. Leaves opposite, often decussate, or whorled, rarely subalternate to alternate, simple, entire; pinnately veined, secondary veins typically joined in a series of intramarginal arches; stipules vestigial or absent. Inflorescences racemes, cymes, or panicles; flowers axillary or terminal, usually 4-, 6- or 8-merous, sometimes 3- or 5-merous, bisexual, regular or irregular. Floral tube perigynous, hemi-epigynous, or epigynous, persistent in fruit, membranous to leathery, often 6-12-ribbed; sepals valvate, equal to much shorter than floral tube, membranous to thickly leathery, persistent; epicalyx alternating with sepals or absent. Petals inserted at rim of floral tube, alternating with sepals, crinkled, clawed or not, frequently caducous, rarely absent. Stamens usually biseriate and 2 × as many as sepals, sometimes uniseriate, inserted near base of floral tube or higher, or numerous, multiseriate, with at least some inserted at floral rim just below sepals (Punica, Sonneratia, and some Duabanga) ; anthers versatile [rarely basifixed]. Ovary superior, half-inferior, or inferior, 2-6- or multi-loculed, with many ovules per locule; style simple; stigma capitate, conic-peltate, or punctiform; placentation axile, sometimes free central at fruit maturity. Fruit partly or completely surrounded by persistent floral tube, loculicidally dehiscent or irregularly dehiscent capsules, infrequently indehiscent, leathery, or berrylike. Seeds usually numerous, without endosperm; embryo straight, cotyledons flat or convolute.

About 31 genera and 625-650 species: widespread in tropical regions, less common in temperate regions; ten genera and 43 species (ten endemic, four introduced) in China.

From the morphological standpoint, the Lythraceae sensu lato (including Trapaceae) have a very generalized morphology, without a single unique, defining character, i.e., there is no morphological synapomorphy that defines the family. At the same time, the genera are distinct. The position of the ovary in Duabanga, Punica, Sonneratia, and Trapa is variable: superior to partly inferior in Sonneratia; partly inferior in Duabanga; and partly, nearly, or completely inferior in Punica and Trapa. Several other features ally these genera to the Lythraceae sensu stricto, including opposite and simple leaves, commonly held wood anatomical characters (true for the Myrtales generally), development of a persistent floral tube, valvate sepals, 4- or 6-merous flowers, introrse and versatile anthers, axile placentation, and seeds without endosperm. Of the four genera, Trapa is the most divergent, but still sufficiently similar to the Lythraceae and Onagraceae to have been considered for membership within either family, or as a closely related family (as has been done in the present Flora) . The inclusion of Sonneratia, Duabanga, and Punica in the Lythraceae adds some additional derived features to the definition of the family, but at the same time, brings together taxa that we know, from molecular sequence data, represent a single historical lineage. That knowledge of evolutionary relationship is lost if the genera are maintained as separate families, whereas the taxonomic utility of the Flora is not affected by their inclusion in an expanded Lythraceae.

The molecular data from four genes (three chloroplast and one nuclear) unquestionably place not only Duabanga, Punica, and Sonneratia, but also Trapa, within the Lythraceae. Punica is well supported as a member of a clade of genera that includes Capuronia Lourteig, Galpinia N. E. Brown, and Pemphis (from East Africa and Madagascar) . Duabanga and Lagerstroemia are sister genera, and Sonneratia and Trapa, as unlikely as it may seem morphologically, are also sister genera. Duabanga, Lagerstroemia, Sonneratia, and Trapa together form one of seven clades in the family.