Arthritis, of which osteoarthritis (OA) is the most common type, is the most frequent cause of disability among adults in the United States. The authors reviewed the epidemiologic literature to identify studies that describe the population-based burden of OA-that is, the burden in all adults in the community. They found that 27 million adults-more than 10% of the U.S. adult population-had clinical OA in 2005, and in 2009 OA was the fourth most common cause of hospitalization. OA is the leading indication for joint replacement surgery; 905,000 knee and hip replacements were performed in 2009 at a cost of $42.3 billion. Obesity is a strong risk factor for OA of the knee and hip. Nurses can improve the quality of life of people with OA by raising awareness among their patients and peers of the substantial OA burden and the strategies, such as physical activity, that can reduce it.