The periodontal probe has been and continues to be used as an important diagnostic instrument by the dental profession. The measurements recorded with the probe have generally been considered to represent a reasonably accurate estimate of sulcus or pocket depth. Recent reports on the histopathology of the periodontal lesion and the histological features of a healing lesion, together with histological studies on the relationship of the probe to periodontal tissues, have shed some new light on periodontal probing. It is now apparent that probing depth measured from the gingival margin seldom corresponds to sulcus or pocket depth. The discrepancy is least in the absence of inflammatory changes and increases with increasing degrees of inflammation. In the presence of periodontitis the probe tip passes through the inflamed tissues to stop at the level of the most coronal intact dento-gingival fibers, approximately 0.3-0.5 mm apical to the apical termination of the junctional epithelium. Decreased probing depth measurements following periodontal therapy may be due in part to decreased penetrability of the gingival tissues by the probe. Following treatment aimed at obtaining new attachment in periodontal defects, wider variations may occur between the location of the probe tip and the most coronal dento-gingival fibers than in the case of untreated sites. This is due in part to the formation of a so-called "long" junctional epithelium. In the absence of inflammation this epithelium may not be penetrable during ordinary probing, but could account for a rapid increase in probing depth measurements when inflammatory changes allow the probe to traverse the epithelium and/or the adjacent infiltrated connective tissue. In view of the difficulty inherent in relating periodontal probing measurements to actual sulcus or pocket depth, the interpretation of periodontal probing in the practice of periodontics may need reappraisal.