Purpose: Rotator cuff disease is the most common shoulder disease. Rotator cuff tear, which is related to cuff tendon degeneration, is commonly encountered in clinical practice. Materials and Methods: Knowledge about the biology of the normal rotator cuff is fundamental to understanding the pathophysiology of and degenerative processes in rotator cuff tendon tears. Furthermore, such basic knowledge provides a rationale for and facilitates the development of treatment modalities. Results and Conclusion: Therefore, we reviewed the biology of the normal rotator cuff tendon, theories to explain the pathophysiology of rotator cuff tendon tear, and current research on apoptosis of rotator cuff tenofibroblasts.