Whereas early classifications of membranoproliferative glomerulonephritis (MPGN) were based on morphologic features, the modern approach is directed at immunofluorescence findings. Glomerular deposits of C3 alone, without immunoglobulin, are the hallmark of alternative complement pathway dysregulation through inherited or acquired defects. These immunoglobulin-negative forms are referred to as C3 glomerulopathy, which encompasses both dense deposit disease and C3 glomerulonephritis. Distinguishing C3 glomerulopathy from immunoglobulin-mediated MPGN is opening the way to better diagnostic, prognostic, and treatment algorithms.