Until recently, membranoproliferative glomerulonephritis (MPGN) was clinically classified as either primary, idiopathic MPGN or as secondary MPGN when an underlying aetiology was identifiable. Primary MPGN was further classified into three types--type I, type II, and type III--based principally on the ultrastructural appearance and location of electron-dense deposits. Both the clinical and histopathologic schemes presented problems, however, as neither was based on disease pathogenesis. An improved understanding of the role of complement in the pathogenesis of MPGN has led to a proposed reclassification into immunoglobulin-mediated disease (driven by the classical complement pathway) and non-immunoglobulin-mediated disease (driven by the alternative complement pathway). This reclassification has led to improved diagnostic clinical algorithms and the emergence of a new grouping of diseases known as the C3 glomerulopathies, best represented by dense deposit disease and C3 glomerulonephritis. In this Review, we re-examine the previous and current classification schemes of MPGN, focusing on the role of complement. We survey current data about the pathogenesis of the C3 glomerulopathies, including familial studies and patient cohorts from the USA and Europe. In addition, we discuss the diagnosis, treatment, and prognosis of the C3 glomerulopathies.