Aggressive periodontitis is characterized by the rapid loss of attachment and bone loss occurring in an otherwise clinically healthy patient, with the amount of microbial deposits inconsistent with disease severity and familial aggregation of diseased individuals. Aggressive periodontitis was formerly classified as early onset periodontitis, i.e. localized juvenile periodontitis (LJP) has been changed to localized aggressive periodontitis (LAP); generalized aggressive periodontitis (GAP) was previously classified as generalized juvenile periodontitis (GJP) and rapidly progressive periodontitis (RPP). This chapter covers the localized aggressive periodontitis/localized juvenile periodontitis, generalized aggressive periodontitis, and risk factors for aggressive forms of periodontitis.