Plants shrubs, sometimes suckering from the roots to form colonies. Stems erect to spreading, unarmed. Leaves opposite. Stipules lacking. Leaf blades simple, unlobed, the margins toothed, glabrous to more commonly pubescent. Inflorescences terminal and sometimes also axillary, short racemes or compound umbellate panicles, sometimes appearing as loose or dense clusters. Flowers more or less epigynous, perfect, actinomorphic, usually subtended by inconspicuous linear bracts, the marginal flowers of the inflorescence sometimes sterile and with enlarged petaloid sepals. Calyces of 4 or 5 distinct sepals. Corollas of 4 or 5 distinct petals. Stamens 8 to numerous, the anthers attached at their bases. Pistil of 3–5 fused carpels. Ovary inferior or partially inferior, with 2–5 locules, with numerous ovules, the placentation axile. Styles 1 or 2 per flower, if solitary then sometimes 4-lobed, the stigmas 1 or 4, if solitary then often 4-lobed, capitate or club-shaped to nearly linear. Fruits capsules (berries elsewhere). Seeds 1 to numerous, often winged. Sixteen or 17 genera, about 170 species, widespread in temperate and subtropical regions of the Northern Hemisphere.

The genera of Hydrangeaceae traditionally were included in a broadly circumscribed Saxifragaceae (Steyermark, 1963). More recently, phylogenetic studies have indicated the woody members of the Saxifragaceae should be moved to several other families (Morgan and Soltis, 1993; Hufford, 1997; Soltis and Soltis, 1997).

In addition to the two genera treated below, the genus Deutzia is frequently cultivated in Missouri. Deutzia scabra Thunb., the most commonly grown species, is a shrub to 2.5 m tall with ascending branches and the leaf blades roughened on both surfaces with minute stellate hairs. The flowers are all fertile and showy with usually white corollas, 3–5-locular ovaries, and 3–5 separate styles. Often the plants have flowers with a doubled perianth. Although this species has not yet been documented to escape in Missouri, it sometimes persists at old home sites and may eventually be recorded from a naturalized population in the state.

Uphof (1922) reported finding Decumaria barbara L. (climbing hydrangea) along a small stream in Carter County, but thus far no specimens have been located to verify any of the species found by this German ecologist during his vegetational studies in southeastern Missouri. This slender liana produces adventitious roots at some nodes and has ovate leaves and umbellate panicles of small flowers similar to those of some Hydrangea species. It differs in its climbing habit and in having all perfect flowers with 7–10 petals. As there are no reports from areas adjacent to the state, this species is presently excluded from the Missouri flora.