The complement system consists of 3 pathways and more than 30 proteins, including those with biological activity that directly or indirectly mediate the effects of this system, plus a set of regulatory proteins necessary to prevent injudicious complement activation on host tissue. The role for complement in the pathogenesis of systemic lupus erythematosus (SLE) is paradoxic. On one hand, the complement system appears to have protective features in that hereditary homozygous deficiencies of classic pathway components are associated with an increased risk for SLE. On the other hand, immune complex-mediated activation of complement in affected tissues is clearly evident in both experimental and human SLE along with pathologic features that are logical consequences of complement activation. By using accurate mouse models of SLE, we have gained remarkable insights into pathogenic features likely relevant to the human disease, and the ability to test potential therapies, some of which have made it to standard clinical use. Studies in genetically altered mice and using recombinant protein inhibitors of complement have confirmed what was believed but unproven-early complement proteins C1q and C4 are protective whereas complement activation later in the pathways is proinflammatory and deleterious. Two complement inhibitors, soluble complement receptor 1 (TP10, Avant Immunotherapeutics, Needham, MA) and a monoclonal anti-C5 antibody (Eculizumab, Alexion Pharmaceuticals, Inc., Cheshire, CT) have been shown to inhibit complement safely and now are being investigated in a variety of clinical conditions. Although these and others earlier in their clinical development hold promise to be used therapeutically in lupus nephritis, this optimism must be tempered by the fact that the clinical trials to prove this remain fraught with obstacles.