The complement system involves both the innate and the adaptive immune systems and has important roles in the pathogenesis of SLE. Complement deficiencies within the classical pathway (C1q, C4 and C2) of activation predispose for development of the autoimmune disease SLE. The association between complement deficiencies and SLE could be explained by several mechanisms, including impaired clearance of immune complexes and impaired handling of apoptotic cells, aberrant tolerance induction or changes in cytokine regulation. Also during SLE disease flares, the complement system is activated giving rise to partial deficiency or dysfunction due to consumption. On the other hand, complement also takes part in the inflammatory reaction in the disease that gives rise to the tissue and organ damage. In this review various aspects of the relation between complement and SLE are discussed.