This document discusses the relationship between diabetes mellitus (DM) and periodontal disease. Uncontrolled DM can lead to microvascular and macrovascular complications and increases susceptibility to infections with poor wound healing. Periodontitis is considered the 6th complication of DM. Bacterial pathogens are altered in the gingival crevicular fluid and blood of diabetics, which contributes to increased severity of periodontal disease. Poorly controlled DM also impairs the functions of polymorphonuclear leukocytes, monocytes, and macrophages that normally help defend against periodontal pathogens. Chronic hyperglycemia in diabetics affects collagen structure and metabolism, reducing periodontal tissue integrity and increasing susceptibility to destruction from infection.