The subscapularis is the largest and most powerful muscle of the rotator cuff, providing up to 50% of the rotator cuff force alone. It plays an essential coordinating role in shoulder function and as an important static and dynamic stabilizer. It is best thought of as two separate muscles, because of two distinct origins, innervation patterns, and insertions. The diagnosis and treatment of posterosuperior rotator cuff tears are well documented in the literature. As knowledge about subscapularis and associated structures has expanded, tears in these areas are increasingly recognized as key sources of pathology.