This document discusses bleeding on probing (BOP) as a diagnostic indicator for periodontal disease. It defines BOP and explains that it represents gingival inflammation in response to dental plaque. BOP has been incorporated into periodontal indices and can provide information on both subject and site-level risk for disease progression based on the percentage of sites that bleed and whether individual sites bleed, respectively. While not a perfect predictor, the presence of BOP in treated patients is a risk factor for increased attachment loss. Standardized probing techniques and regular periodontal maintenance visits are important for reliable use of BOP in monitoring periodontal disease.