The natural history and treatment of degenerative arthritis of the knee in active patients is a topic of great interest, with continually evolving concepts and techniques. Osteoarthritis is a spectrum of clinical entities, ranging from focal chondral defects to established arthrosis resulting from biologic and biomechanical hyaline cartilage failure. Evaluation of the active patient with knee arthritis should include a comprehensive history emphasizing symptom manifestation, activity level, and previous surgical treatment. The physical examination must include an evaluation of extremity alignment, gait patterns, and coexisting disorders of the spine and adjacent joints. Diagnostic testing is usually straightforward and should include the 45-degree flexion weight-bearing posteroanterior plain radiograph. Nonsurgical treatment modalities include rehabilitation, lifestyle modification, bracing, supportive devices, and medical management, including use of the new chondroprotective oral and injectable agents. Several surgical options exist, each with specific indications. Arthroscopic debridement can provide a positive, but often short-lived, reduction in the severity of symptoms. Tibial or femoral osteotomy may maintain the patient's active lifestyle and delay the need for arthroplasty. Unicompartmental and total knee arthroplasty can each provide reliable relief of symptoms but may not permit a return to the activities that the patient values.