This paper, after presenting a brief case for the relevance of root caries to today's practitioner, describes the differing diagnostic needs of clinical practitioners and clinical researchers. The goal is to inform the clinician of the state-of-the-art that exists today for the diagnosis of root caries in both clinical and research settings. Differing definitions for the diagnosis of root caries are presented that are useful, respectively, for the clinician and for the clinical researcher. Reasons for why there are differing definitions for clinicians and researchers are presented. Finally, eight critical issues are presented that will have to be resolved if clinical research on root caries is to improve its rigor: (1) active vs. inactive lesions, (2) supra- vs. sub-gingival lesions, (3) lesions crossing the CEJ, (4) prior clinical treatment: a visibility issue, (5) oral debris: a visibility issue, (6) instruments for tactile clues, (7) radiographs, and (8) diagnostic conventions.