A circuit theory for metallic single-electron tunnelling (SET) junctions is presented. In detail circuits with a single SET junction in arbitrary environments are described. Based on the conservation of energy in the circuits-a fundamental circuit theorem-equivalent circuit elements are proposed and possible physical justifications are presented. The resulting model represents the tunnel event by an impulse current source, the junction by a charged capacitor, and the tunnelling condition as a discrete process based on local circuit parameters-and may include a tunnelling time. Simple examples illustrate Coulomb blockade, Coulomb oscillations, and continuous direct tunnelling. Copyright (C) 2007 John Wiley & Sons, Ltd.