Periodontal diseases are bacterial infections that occur at or below the gum line. In contrast to gingivitis, which affects only the gums, periodontitis (severe periodontal disease) may involve the soft tissue and bone supporting the teeth. An estimated 70 percent of the adult U.S. population is affected by these infections. This includes 20-30 percent who have periodontitis that threatens the loss of teeth. Approximately $5 billion is spent on treatment of periodontal diseases each year. This report deals with the treatment of chronic periodontitis in adults. The key question is whether scaling and root planing (SRP) accompanied by an antimicrobial agent, as a supplemental or adjunct treatment, results in improved outcomes that persist over time in adults with chronic periodontitis when compared with SRP alone (or SRP and placebo). The primary outcomes of interest in this report are reductions in probing depth (PD) and gains in clinical attachment level (CAL). Of secondary interest are reductions in selected disease-causing bacteria, particularly reduction in the percentage of spirochetes present in dental plaque or in fluid from the gingival crevice.