Complement is an important aspect of defence against infection and its activation and regulation are finely balanced. Disordered complement regulation can lead to C3 glomerulonephritis (C3GN), which is characterized by complement (but not immunoglobulin) deposition in the glomerulus of the kidney. Although only recently recognized as a clinical entity, C3GN is important and elucidation of its molecular causes, by studies of single cases and families, has identified key proteins that protect the kidney from complement-mediated damage. The commonest cause of C3GN is complement factor H-related 5 (CFHR5) nephropathy, which is endemic in Greek Cypriots. Genetic evidence implicates some of the same complement regulators in the aetiology of common immune complex glomerular disorders such as IgA nephropathy and lupus nephritis. Importantly, therapeutic manipulation of the complement pathway is now feasible. An exciting challenge is to determine whether this can be applied to kidney diseases that are caused by complement dysregulation, and also whether they might be used to intervene in other kidney diseases.