Tendons are complex composite material composed primarily of water, collagen, proteolycans, and cells, designed to transmit tensile loads from muscle to bone. Although rotator cuff tendons differ in many ways from other tendons in the body, a knowledge of basic tendon structure and function is helpful in understanding rotator cuff tendon biology, injury, and repair. In addition to type I collagen, rotator cuff tendons contain small amounts of type III collagen, which play a role in healing and repair. In comparison with other tendons, the increased glycosaminoglycan and proteoglycan content seen in rotator cuff tendons may be adaptive, pathologic, or both. The etiology of rotator cuff pathology is probably related to trauma, aging, and degeneration. As our understanding of these processes increases, we will be able to develop and implement improved preventative and therapeutic interventions for rotator cuff pathology.