Background: The glenohumeral joint combines large range of motion and insufficient bony stabilization, making it susceptible to instability and dislocations. Arthroscopic surgery is routinely used as a diagnostic tool and has been considered the gold standard for the diagnosis of shoulder lesions. However, several studies have demonstrated variability in intraobserver and interobserver agreement. Purpose: To evaluate interobserver and intraobserver agreement in the assessment of intra-articular lesions associated with shoulder instability among fellowship-trained shoulder surgeons. Study Design: Cohort study (diagnosis); Level of evidence, 3. Methods: A total of 24 arthroscopic videos from patients treated for recurrent shoulder instability were shown to a group of 10 fellowship-trained shoulder surgeons who are members of the Multicenter Orthopaedic Outcomes Network (MOON) Shoulder Group. They were presented to the surgeons on 2 different occasions at least 2 months apart. They were asked to classify labral tears by their position, type, extension, other intra-articular abnormality, and preferred treatment. No patient history or physical examination data were provided. The primary outcome was the median overall percentage of agreement for the surgeons performing a video review, measured for each variable evaluated. Intraclass correlation coefficients were used to evaluate continuous variables, and kappa values were used for categorical items. Results: Interobserver agreement was good for anterior labral lesions; good for Hill-Sachs lesions; and moderate for lesions of the superior labrum, posterior labrum, anterior sublabral foramen, and position and extension of the tear. Intraobserver agreement was either good or very good for all variables evaluated, except for being poor for inferior labral lesions and moderate for lesions of the meniscoid superior labrum. Conclusion: Interobserver and intraobserver reliability for the arthroscopic assessment of labral tears in patients with recurrent shoulder instability were good to moderate for the majority of anatomic structures assessed. There was relatively good agreement between shoulder instability surgeons on assessing and documenting shoulder instability–associated abnormalities. These findings are important when interpreting collaborative clinical cohort studies with numerous surgeons involved in the research.