The etiology of SLE is multifactorial with an important genetic impact. Several genes involved in control of autoimmunity and inflammation appear to be important. Hereditary complement deficiency states are associated with increased risk of SLE, but contribute only marginally to the incidence of SLE in the population. However, these conditions have contributed considerably to the knowledge of pathogenetic mechanisms in this disease. Furthermore, acquired complement deficiency is a common finding in SLE. Complement has important protective functions but also contributes to tissue damage. Measurement of classical pathway complement components is important in the diagnosis of SLE and for monitoring of immune complex mediated manifestations, especially proliferative glomerulonephritis. New complement activation tests, although promising in studies of selected patient groups, have not yet been proven to be of clinical value.