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Context: Athletes who participate in throwing and racket sports consistently demonstrate adaptive changes in glenohumeral-joint internal and external rotation in the dominant arm.Measurements of these motions have demonstrated excellent intrarater and poor interrater reliability.Objective: To determine intrarater reliability, interrater reliability, and standard error of measurement for shoulder internal rotation, external rotation, and total arc of motion using an inclinometer in 3 testing procedures in National Collegiate Athletic Association Division I baseball and softball athletes.Design: Cross-sectional study. Setting: Athletic department. Patients or Other Participants:Thirty-eight players participated in the study. Shoulder internal rotation, external rotation, and total arc of motion were measured by 2 investigators in 3 test positions. The standard supine position was compared with a side-lying test position, as well as a supine test position without examiner overpressure.Results: Excellent intrarater reliability was noted for all 3 test positions and ranges of motion, with intraclass correlation coefficient values ranging from 0.93 to 0.99. Results for interrater reliability were less favorable. Reliability for internal rotation was highest in the side-lying position (0.68) and reliability for external rotation and total arc was highest in the supine-without-overpressure position (0.774 and 0.713, respectively). The supine-with-overpressure position yielded the lowest interrater reliability results in all positions. The side-lying position had the most consistent results, with very little variation among intraclass correlation coefficient values for the various test positions.Conclusions: The results of our study clearly indicate that the side-lying test procedure is of equal or greater value than the traditional supine-with-overpressure method.Key Words: shoulder, glenohumeral internal-rotation deficit, inclinometer, baseball, softball, range of motion Key PointsOverhead athletes have been shown to demonstrate adaptive changes in glenohumeral internal and external rotation in the dominant arm. To make decisions about appropriate interventions, clinicians must be able to accurately assess glenohumeral rotation. For all 3 test positions, the side-lying test procedure demonstrated excellent reliability and, thus, is of equal or greater value than the traditional supine-with-overpressure method.