The complement cascade is part of the innate immune system, and it provides an important line of defense against infections. Inappropriate or excessive activation of this system, however, is also a driver of kidney inflammation in many diseases. Complement inhibitory drugs have now been approved by the US Food and Drug Administration for the treatment of 5 renal diseases: atypical hemolytic uremic syndrome, C3 glomerulopathy, immune-complex membranoproliferative glomerulonephritis, antineutrophil cytoplasmic antibody–associated vasculitis, and IgA nephropathy.