The vast majority of cultivars of A. cepa belong to the "common onion group" (A. cepa var. cepa) and are usually referred to simply as "onions". The Aggregatum Group of cultivars (A. cepa var. aggregatum) includes both shallots and potato onions.
Allium cepa is known exclusively in cultivation, but related wild species occur in Central Asia. The most closely related species include Allium vavilovii (Popov & Vved.) and Allium asarense (R.M. Fritsch & Matin) from Iran. However, Zohary and Hopf warn that "there are doubts whether the A. vavilovii collections tested represent genuine wild material or only feral derivatives of the crop."