Data on interrater reliability in assessing a number of clinical signs commonly evaluated in the diagnosis and treatment of temporomandibular disorders (TMD) is presented in this article. Four experienced dental hygienists who were field examiners for a large epidemiologic study of TMD and three experienced clinical TMD specialists (dentists) who are coinvestigators in the same study followed carefully detailed specifications and criteria for examination of TMD patients and pain-free controls. Excellent reliability was found for vertical range of motion measures and for summary indices measuring the overall presence of a clinical sign that could arise from several sources (for example, summary indices of muscle palpation pain). However, many clinical signs important in the differential diagnosis of subtypes of TMD were not measured with high reliability. In particular, assessment of pain in response to muscle palpation and identification of specific temporomandibular joint sounds seemed to be possible only with modest, sometimes marginal, reliability. These modest reliabilities could arise from examiner error because the clinical signs are themselves unreliable, changing spontaneously over time and making it difficult to find the same sign on successive examinations. The finding that, without calibration, experienced clinicians showed low reliability with other clinicians suggests the importance of establishing reliable clinical standards for the examination and diagnostic classification of TMD.