The Craniomandibular Index (CMI) was developed to provide a standardized measure of severity of problems in mandibular movement, TMJ noise, and muscle and joint tenderness for use in epidemiological and clinical outcome studies. The instrument was designed to have clearly defined objective criteria, simple clinical methods, and ease in scoring; it is divided into the Dysfunction Index and the Palpation Index. Inter-rater reliability (three raters) and intra-rater reliability (19 patients examined twice by one rater) were tested to determine whether the instrument has operational definitions sufficiently precise to allow for consistency in use between different raters and with one rater over time. Intraclass Correlation Coefficient for inter-rater reliability was 0.84 for the Dysfunction Index, 0.87 for the Palpation Index, and 0.95 for the CMI. Correlation for intra-rater reliability was 0.92 for the Dysfunction Index, 0.86 for the Palpation Index, and 0.96 for the CMI. These results support the reliability of the CMI for use in epidemiological and clinical studies. Users are cautioned about the subjectivity of numerous items within the CMI and the strict methodological guidelines that must be followed in order to assure accuracy and reproducibility of results.