Gingival crevicular fluid, also called GCF, was recognized as an important factor in diagnosing periodontal disease over six decades ago. It is an inflammatory exudate that is derived from periodontal tissue and contains primarily the byproducts of tissue breakdown, antibodies, inflammatory mediators, and serum. It also contains structural periodontal cells, leukocytes, and normal oral bacteria that is usually present in the mouth. The serum component of this fluid is mostly derived from postcapillary venules, or microvascular, leakage. This fluid has an important role in maintaining the antimicrobial defense of the periodontium and maintaining the structure of the junctional epithelium. The bacteria most often responsible for periodontal disease are Treponema denticola and Porphyromonas gingivalis. The bacteria create broad-spectrum neutral proteinases to attack healthy tissue, which are then found in both samples of gingival crevicular fluid and plaque in patients who have periodontal disease. Patients with periodontal disease have an excess of GCF, whereas patients with healthy gums have very little.