Osteoarthritis (OA), also termed degenerative joint disease, is the most common arthritis. There are two main forms, primary and secondary. Primary OA is an age-related disease with a genetic susceptibility in some individuals to earlier onset and more rapid progression. It occurs in joints without local predisposing factors. Primary OA may be localized or generalized. OA is a complex interaction of advancing age, genetic predisposition, mechanical stress, obesity, as well as metabolic and biochemical factors all of which may affect the degree, extent, and progression of disease. Obesity is the only modifiable risk factor. Primary OA increases significantly in prevalence after the age of 50 with the majority of individuals demonstrating some form of OA after the age of 70 years.