The shoulder complex is one of the most commonly affected regions for which individuals present to physical therapy. Numerous shoulder disorders result in altered mobility with posterior shoulder tightness (PST), impaired internal rotation (IR), and either decreased or increased external rotation (ER) often reported in research investigations. The accurate assessment of shoulder mobility is an integral component of both the physical therapy examination and intervention. Therefore, the reliability and sensitivity to change of instruments used to measure mobility must be established. The purpose of this study was to investigate the intrarater reliability and minimal detectable change (MDC(90)) of inclinometric measurements designed to quantify shoulder mobility. Active shoulder IR, ER, and passive PST were measured on the nondominant side of 30 asymptomatic volunteers in an intersession design. Intraclass correlation coefficients (ICCs) using model 3, k were excellent using the protocols described in this investigation with IR = 0.987, ER = 0.970, and PST = 0.964. The MDC(90) indicates that a change of greater than or equal to 4 degrees (IR), 5 degrees (ER), and 8 degrees (PST) would be required to be 90% certain that the change is not due to intertrial variability or measurement error.