Osteoarthritis is the most common joint disease, and a leading cause of pain and physical disability in older people. It is also associated with a major disease burden in middle-aged people, and represents 4–5% of disability-adjusted life-years in those aged 30–60 years. The burden of disease caused by osteoarthritis is greater for women than for men, and is projected to increase as the population ages. Joint damage in osteoarthritis is caused by several predisposing systemic factors (including genetics), and local mechanical factors (including joint injury and load) that determine its distribution and severity. The knee, hand, and hip are most often affected. Joint damage might be associated with use-related joint pain and restricted movement. Diagnostic criteria are ambiguous and disease assessment is difficult: for example, pain and radiographical severity of joint damage are only weakly associated. Socioeconomic and psychosocial factors have an important role in establishing disease burden for the individual with osteoarthritis, as in many other musculoskeletal diseases.