Shoulder instability (SI) is common and multifactorial, affecting both the general population and athletes, with a lifetime prevalence of 1%–2% and an incidence of 0.12/1000 sports injuries. It occurs when the humeral head moves beyond the glenoid margins, compromising stability and function. SI can be traumatic, often involving dislocation or subluxation (TUBS: Traumatic, Unilateral, Bankart lesion, Surgery), or atraumatic, related to congenital or acquired laxity (AMBRI: Atraumatic, Multidirectional, Bilateral, Rehabilitation, Inferior capsular shift). Its pathophysiology involves static stabilizers—capsule, labrum, glenohumeral ligaments—and dynamic stabilizers, mainly the rotator cuff and periscapular muscles. Diagnosis combines medical history, clinical assessment, and imaging. Tests like apprehension, relocation, Jerk, and Kim provide direction-specific insights, while the Beighton score assesses laxity. Radiographs, computed tomography (CT), magnetic resonance imaging (MRI), and MR arthrography detect lesions like Bankart or Hill-Sachs. Classification systems—including macro- vs. micro-instability, TUBS/AMBRI, multidirectional instability (MDI), functional shoulder instability (FSI), and the Stanmore triangle—help with diagnosis and management, reflecting the heterogeneity of this entity. Treatment varies by cause, age, activity, and recurrence risk. Arthroscopic stabilization is effective for recurrent traumatic cases, while physiotherapy is key for atraumatic and functional instability.