Dysregulation of the alternative pathway of complement plays a central role in the pathophysiology of C3 Glomerulopathy (C3G). Various autoimmune and genetic factors targeting the alternative pathway have been associated to both C3G and primary Immunoglobulin-associated Membranoproliferative Glomerulonephritis (Ig-MPGN), suggesting shared pathophysiological mechanisms. This review highlights the wide range of disease drivers identified that mainly target components or protein complexes of the alternative pathway, both in C3G and Ig-MPGN. Nephritic factors, which constitute a heterogeneous group of autoantibodies targeting the C3 or the C5 convertase, are the most common abnormalities. Monoclonal gammopathies are frequent in aging adults. They may promote complement activation and have in some cases also found to target alternative pathway regulatory proteins. Additionally, some patients with C3G and Ig-MPGN carry rare variants in genes encoding complement activating or regulating proteins of the alternative pathway. This review provides an informative overview of pathogenetic mechanisms associated with each abnormality, acting at different steps in the complement cascade. The diversity of targets involved in the C3G pathophysiology suggests the potential benefit of therapeutical approaches tailored to the underlying disease drivers, with a pivotal impact upstream or at the level of the C3 or C5 convertase activity.