The purpose of this study was to measure the reliability, validity, and responsiveness of the Simple Shoulder Test (SST) scale and to examine these in patients stratified by age and injury type. Test-retest reliability, content validity, criterion validity, construct validity, and responsiveness were determined for the SST. The study population comprised 1077 patients with shoulder instability and rotator cuff injuries, ranging in age from 14 to 85 years. The SST demonstrated acceptable test-retest reliability (intraclass correlation coefficient >0.90) and content validity (floor and ceiling effects <10%). Correlations with the physical functioning component of the Short Form 12 were significant (r = 0.439, P < .05); however, the correlations were not significant when stratified by age group (>60 years) (r = 0.271, P = .349) and injury type (rotator cuff injury) (r = 0.337, P = .085). Correlations with the American Shoulder and Elbow Surgeons were also significant (r = 0.807, P < .001). The construct validity of the SST was acceptable, with all 8 hypotheses demonstrating significance (P < .05). The SST was responsive to change (effect size, 0.81; standardized response mean, 0.81). However, there were differences after stratification for age group and injury type. The SST demonstrated overall acceptable psychometric performance; however, differences were found when data were stratified by age and injury type.