Chestnuts belong to the family Fagaceae, which also includes oaks and beeches. The four main species groups are commonly known as American, European, Chinese, and Japanese chestnuts.
- The two accepted species of American chestnuts are Castanea dentata (American chestnut – eastern states) and Castanea pumila (American or Allegheny chinkapin, also known as "dwarf chestnut" – southern and eastern states).
- Asian chestnuts include Castanea mollissima (Chinese chestnut),
Castanea henryi (Chinese chinkapin, also called Henry's chestnut – China),
Castanea seguinii (also called Seguin's chestnut – China) and Castanea crenata (Japanese chestnut, Korean chestnut). A tropical version of chestnut trees can reach 20–30 m with fruits or seeds half the size of the Chinese version. It is edible and taste like C. mollissima. It is found in Malaysia and perhaps other Southeast Asian countries, as well. Perhaps because its seeds are relatively small, it is not commercially cultivated.
- The European chestnut, Castanea sativa (sweet chestnut; also called "Spanish chestnut" in the US and the UK), is the only European species of chestnut, though it was successfully introduced to the Himalayas and other temperate parts of Asia.
Chestnuts should not be confused with horse chestnuts, in the genus Aesculus, which are not related to true chestnuts but are named for producing nuts of similar appearance that are mildly poisonous to humans. Nor should they be confused with water chestnuts, which are tubers of an aquatic herbaceous plant in the sedge family Cyperaceae.
Other trees commonly mistaken for chestnut trees are the chestnut oak (Quercus prinus) and the American beech (Fagus grandifolia), both of which are also in the Fagaceae.