Osteoarthritis of the knee is not only common, but is also responsible for a significant amount of disability in the aging population. Although the exact etiology of this disease process is not known, osteoarthritis has been well described. The primary site affected by osteoarthritis is the articular cartilage. An understanding of the surrounding anatomy, cartilage structure and function, and the response of cartilage to injury will aid both the clinician and researcher in looking for ways to interrupt or retard the degenerative process of osteoarthritis.