Osteoarthritis (OA) affects about 21 million people in the United States. By age 65, the majority of the population has radiographic evidence of osteoarthritis and 11 percent have symptomatic OA of the knee. This is a systematic review of three treatments for osteoarthritis (OA) of the knee: intraarticular injections of viscosupplements; oral glucosamine, chondroitin or the combination; and, arthroscopic lavage and debridement. The key questions are: (1) effectiveness and harms in primary OA of the knee, (2) in secondary OA of the knee, (3) in subpopulations, and (4) comparison of the three interventions. This section outlines the burden of illness and clinical management of osteoarthritis of the knee, the interventions of interest and uncertainties, and overviews key questions to be addressed. According to the U.S. Centers for Disease Control and Prevention (CDC), an estimated 22 percent of adults (46 million) in the United States have doctor-diagnosed arthritis. Earlier figures suggest approximately 11 percent of the population 64 years and older has symptomatic OA of the knee. Symptoms of OA typically begin after age 40 and progress slowly, with radiographic evidence of the disease present in the majority of the population by 65 years of age and in approximately 80 percent of the population age 75 years and older. OA of the knee is more common in women than in men, with risk factors that include obesity, previous knee injury or surgery, and occupational bending and lifting. Loss of joint function as a result of OA overall is a major cause of work disability and reduced quality of life.