Background: Anterior shoulder instability is a common complaint of young athletes. Posterior instability in this population is less well understood, and the standard of care has not been defined. The purpose of the study is to compare index frequency, treatment choice, and athlete disability following an incident of anterior or posterior shoulder instability in high school and collegiate athletes. Methods: A total of 58 high school and collegiate athletes (n=30 athletes with anterior instability; n=28 athletes with posterior instability) were included. Athletes suffering from a traumatic sport-related shoulder instability episode during a team-sponsored practice or game were identified by their school athletic trainer. Athletes were referred to the sports medicine physician or orthopedic surgeon for diagnosis and initial treatment choice (operative vs. nonoperative). Athletes diagnosed with traumatic anterior or posterior instability who completed the full course of treatment and provided pre- and post-treatment patient-reported outcome measures were included in the study. The frequency of shoulder instability was compared by direction, mechanism of injury (MOI), and treatment choice through χ2 analyses. A repeated measures analysis of variance was used to compare the functional outcomes by treatment type and direction of instability (α = 0.05). Results: Athletes diagnosed with anterior instability were more likely to report a chief complaint of instability (70%), whereas those diagnosed with posterior instability reported a primary complaint of pain interfering with function (96%) (P = .001). The primary MOI classified as a contact event was similar between anterior and posterior instability groups (77% vs. 54%, P = .06) as well as the decision to proceed with surgery (60% vs. 72%, P = .31). In patients with nonoperative care, athletes with anterior instability had significantly more initial disability than those with posterior instability (32±6.1 vs. 58±8.1, P = .001). Pre- and post-treatment Penn Shoulder Scores for athletes treated with early surgery were similar (P > .05). There were no differences in functional outcomes at discharge in those treated nonoperatively regardless of direction of instability (P = .24); however, change in Penn score was significantly greater in those with anterior (61±18.7) than those with posterior (27 ± 25.2) instability (P = .002). Conclusion: Athletes with anterior instability appear to have different mechanisms and complaints than those with posterior instability. Among those that receive nonoperative treatment, athletes with anterior instability have significantly greater initial disability and change in disability than those with posterior disability during course of care.