Since their advent in 2001, virtual power plant (VPP) auctions have been implemented widely. In this paper, we describe the simultaneous ascending-clock auction format that has been used for virtually all VPP auctions to date, elaborating on other design choices that most VPP auctions have had in common as well as discussing a few aspects that have varied significantly among VPP auctions. We then evaluate the various objectives of regulators in requiring VPP auctions, concluding that the auctions have been effective devices for facilitating new entry into electricity markets and for developing wholesale power markets.