Hippocastanaceae is the name given to a small group of trees and shrubs, when this group is treated as a family. Its most widespread genus is Aesculus

Aesculus

The genus Aesculus comprises 13-19 species of woody trees and shrubs native to the temperate northern hemisphere, with 6 species native to North America and 7-13 species native to Eurasia; there are also several hybrids. Species are deciduous or evergreen...

(the horse-chestnuts and buckeyes, syn. Pavia). However, the American genus Billia

Billia

Billia is a genus of two species of tree native to the Americas, from central Mexico to Ecuador. The wood is used in carpentry....

and the Chinese genus Handeliodendron

Handeliodendron

Handeliodendron consists of a single species of deciduous tree/shrub native to China, and is classified as rare.- Description :It grows up to 15 meters tall, and grows in mountain areas characterized by irregular limestone formations in Guangxi and Guizhou...

are also sometimes included in this family. A feature of the family is the palmate compound leaves.

Members of this family are closely related to the large, mostly tropical family Sapindaceae

Sapindaceae

Sapindaceae, also known as the soapberry family, is a family of flowering plants in the order Sapindales. There are about 140-150 genera with 1400-2000 species, including maple, horse chestnut and lychee....

, and some systems of plant taxonomy include the members of the Hippocastanaceae, along with members of the Aceraceae, in an enlarged family Sapindaceae.
Recent molecular phylogenetic

Molecular phylogeny

Molecular phylogenetics is the analysis of hereditary molecular differences, mainly in DNA sequences, to gain information on an organism's evolutionary relationships. The result of a molecular phylogenetic analysis is expressed in a phylogenetic tree...

 research (Harrington et al. 2005) has shown that while both Aceraceae and Hippocastanaceae are monophyletic

Monophyly

In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...

 in themselves, their removal from Sapindaceae sensu lato would leave Sapindaceae sensu stricto as a paraphyletic

Paraphyly

A group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...

 group, particularly with reference to the genus Xanthoceras

Xanthoceras

Xanthoceras sorbifolium , the sole species in the genus Xanthoceras, is a flowering plant in the family Sapindaceae, native to northern China in the provinces of Gansu, Hebei, Henan, Liaoning, Nei Monggol, Ningxia, Shaanxi, and Shandong.It is a large shrub or small tree growing to 8 m tall...

. Therefore it is now usually synonymized with subfamily Hippocastanoideae

Hippocastanoideae

Hippocastanoideae is a subfamily of flowering plants in the soapberry family, Sapindaceae.-Genera:*Acer L. - Maples*Aesculus L. - Horse-chestnuts, Buckeyes*Billia Peyr.*Dipteronia Oliv....

.