Betulaceae, the birch family, includes six genera of deciduous nut-bearing and , including the , , , , Ostryopsis, and Ostrya numbering a total of 167 species. They are mostly natives of the temperate Northern Hemisphere, with a few species reaching the Southern Hemisphere in the Andes in South America. Their typical flowers are catkins and often appear before leaves.
In the past, the family was often divided into two families, Betulaceae ( Alnus, Betula) and Corylaceae (the rest). Recent treatments, including the Angiosperm Phylogeny Group, have described these two groups as subfamilies within an expanded Betulaceae: Betuloideae and Coryloideae.
Betulaceae flowers are monoecious, meaning that they have both male and female flowers on the same tree. Their flowers present as catkins and are small and inconspicuous, often with reduced perianth parts. These flowers have large feathery stamen and produce a high volume of pollen, as they rely on wind pollination. Their leaves are simple, with alternate arrangement and doubly serrate margins.
According to molecular phylogeny, the closest relatives of the Betulaceae are the Casuarinaceae, or the she-oaks.
The other genera include a number of popular ornamental plant, widely planted in parks and large gardens; several of the birches are particularly valued for their smooth, brightly coloured bark.
The wood is generally hard, tough and heavy, particularly so; several species were of significant importance in the past where very hard wood capable of withstanding heavy wear was required, such as for , , gear, tool handles, chopping boards, and wooden pegs. In most of these uses, wood has now been replaced by metal or other man-made materials.