Voltage-gated calcium channels consist of the main pore-forming α(1) subunit, together, except in the case of the T-type channels, with β and α(2)δ and sometimes γ subunits, which are collectively termed auxiliary or accessory subunits. This review will concentrate on the properties and role of the α(2)δ subunits of these channels. These proteins are largely extracellular, membrane-associated proteins which influence the trafficking, localization, and biophysical properties of the channels. This article is part of a Special Issue entitled: Calcium channels.