"Betulaceae belong to an ancient lineage, traceable in the fossil record to the upper Cretaceous. They are easily distinguished by their woody habit; simple, pinnately veined, usually ovate, sharp-toothed leaves; long staminate catkins that often develop the season before anthesis; and (except in Corylus and Ostryopsis ) strobiluslike infructescences. The family is held together on the basis of many features, including habit, leaf structure and arrangement, trichome morphology, wood anatomy, inflorescence morphology, ovary and ovule structure, pollen morphology, embryo structure, and fertilization and germination patterns. Five of the six constituent genera inhabit the boreal and cool temperate zones of Eurasia, North America, and the mountains of Mexico and Central America, and two genera also grow in the Andes to northern Argentina in South America. The remaining genus, Ostryopsis Decaisne (most closely related to Corylus Linnaeus), consists of two species of shrubs restricted to northern and western China.

The group is sometimes divided into two families, Betulaceae ( Alnus and Betula ) and Corylaceae ( Carpinus , Ostrya , Corylus , and Ostryopsis ), especially in Europe (e.g., H. K. Airy Shaw in J. C. Willis 1973; R. M. T. Dahlgren 1980; J. Hutchinson 1959). In America, this treatment has been followed by A. J. Rehder (1940), J. K. Small (1903), and a few others. Some of those writers have based their recognition of two families in part on the belief that a fundamental difference exists in the staminate inflorescences of the two groups. This view is no longer widely accepted, and most modern authors maintain the family as a single group composed of two subfamilies, Betuloideae and Coryloideae." -- eFloras.org, Flora of North America