The genus Quercus contains about 500 known species, plus about 180 hybrids between them. The genus, as is the case with many large genera, is divided into subgenera and sections. Traditionally, the genus Quercus was divided into the two subgenera Cyclobalanopsis, the ring-cupped oaks, and Quercus, which included all the other sections. However, a comprehensive revision in 2017 identified different relationships. Now the genus is commonly divided into a subgenus Quercus and a subgenus Cerris, with Cyclobalanopsis included in the latter. The sections of subgenus Quercus are mostly native to the New World, with the notable exception of the white oaks of sect. Quercus and the endemic Quercus pontica. In contrast, the sections of the subgenus Cerris are exclusively native to the Old World. Unless otherwise indicated, the lists which follow contain all the species accepted by Plants of the World Online, plus selected hybrids that are also accepted, with placement into sections based on a list produced by Denk et al. for their 2017 classification of the genus.

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Species with evergreen foliage ("live oaks") are tagged '#'. Species in the genus have been recategorized between deciduous and evergreen on numerous occasions, although this does not necessarily mean that species in the two groups are closely related.

Subgenus Quercus

Section Quercus

Section Mesobalanus was included in section Quercus in the 2017 classification used here. Other synonyms include Q. sect. Albae and Q. sect. Macrocarpae. The section comprises the white oaks from Europe, Asia, north Africa, Central and North America. Styles short; acorns mature in 6 months, sweet or slightly bitter, inside of acorn shell hairless.

Section Ponticae

Species are native to Western Asia and Western North America. They produce catkins up to 10cm long; the acorns mature annually.

Section Protobalanus

The intermediate oaks. Southwest USA and northwest Mexico. Styles short, acorns mature in 18 months, very bitter, inside of acorn shell woolly.

Section Lobatae

The red oaks (synonym sect. Erythrobalanus), native to North, Central and South America. Styles long, acorns mature in 18 months (in most species), very bitter, inside of acorn shell woolly.

Section Virentes

Section Virentes has also been treated at lower ranks. Species are native south-eastern Northern America, Mexico, the West Indies (Cuba), and Central America. A 2017 classification included seven species:

Subgenus Cerris

Section Cerris

Species are native to Europe, north Africa and Asia. Styles long; acorns mature in 18 months, very bitter, inside of acorn shell hairless or slightly hairy.

Section Ilex

Species in section Ilex are native to Eurasia and northern Africa. Styles medium-long; acorns mature in 12–24 months, appearing hairy on the inside. Evergreen leaves, with bristle-like extensions on the teeth. (Sister group to sect. Cerris and sometimes included in it.)

Section Cyclobalanopsis

The ring-cupped oaks (synonym genus Cyclobalanopsis), native to eastern and southeastern tropical Asia. They have corns with distinctive cups bearing concrescent rings of scales. They commonly also have densely clustered acorns, though this does not apply to all of the species. About 90 species.

Section uncertain

Intersectional hybrids