Periodontitis in humans can be divided into two clinical forms: aggressive and chronic periodontitis. Recent studies on the microbial composition of subgingival plaque reveal no major differences but for a slightly higher prevalence of Porphyromonas gingivalis and Actinobacillus actinomycetemcomitans in the aggressive form. Treatment of both forms involves mechanical supra- and subgingival debridement and periodontal surgery on indication. Recently, it has been shown that adjunctive systemic antimicrobial therapy with metronidazole and amoxicilline significantly improves clinical treatment outcome in patients with generalized aggressive periodontitis. Microbial diagnosis appears to be a useful procedure in determining the most efficient use of antibiotics in treating patients with periodontitis.