Several distinct pathological patterns of glomerular inflammation are associated with abnormal regulation of the complement system, specifically, with dysregulation of the alternative pathway of the complement system. However, these conditions share the pathological finding of complement C3 (C3) deposited within the glomerulus in the absence of substantial immunoglobulin. This finding has alerted us and others to the possible presence of genetic and acquired complement dysregulation in individual patients. This article summarizes our current understanding of the relationship between dysregulation of the complement system and glomerular inflammation. Here, we suggest that glomerular pathologies that are characterized by the isolated deposition of C3 could usefully be classified by the term C3 glomerulopathy. In our view, this classification would alert the pathologist and nephrologist to the importance of screening for acquired and genetic abnormalities in complement regulation. In the future, it could help to identify individuals who might benefit from therapeutic inhibition of the complement system.