Aster (syn. Diplopappus Cass.) is a genus of flowering plants in the family Asteraceae. The genus once contained nearly 600 species in Eurasia and North America, but after morphologic and molecular research on the genus during the 1990s, it was decided that the North American species are better treated in a series of other related genera. After this split there are roughly 180 species within the genus, all but one being confined to Eurasia. The name Aster comes from the Ancient Greek word ἀστήρ (astér), meaning "star", referring to the shape of the flower head. Many species and a variety of hybrids and varieties are popular as garden plants because of their attractive and colourful flowers. Aster species are used as food plants by the larvae of a number of Lepidoptera species—see list of Lepidoptera that feed on Aster. Asters can grow in all hardiness zones.
The genus Aster is now generally restricted to the Old World species, with Aster amellus being the type species of the genus, as well as of the family Asteraceae. The New World species have now been reclassified in the genera Almutaster, Canadanthus, Doellingeria, Eucephalus, Eurybia, Ionactis, Oligoneuron, Oreostemma, Sericocarpus and Symphyotrichum, though all are treated within the tribe Astereae. Regardless of the taxonomic change, all are still widely referred to as "asters" in the horticultural trades. See the List of Aster synonyms for more information.
In the UK there are only two native members of the genus: Goldilocks, which is very rare, and Aster tripolium, the Sea aster. Aster alpinus spp. vierhapperi is the only species native to North America.