Osteoarthrosis of the knee in people < 45 years of age is treated according to the same guidelines as used for people ≥ 45 years. This review gives a summary of these guidelines. Treatment is divided into three phases. Phase 1: supervised exercise therapy and weight loss when indicated. Phase 2: treatment with painkillers, unloading braces can be con-sidered in case of unicompartmental osteoarthrosis. If there is insufficient effect of Phase 1 and phase 2, patients should be referred for orthopaedic consultation. Phase 3: surgical treatment including unloading osteotomy, unicompartmental or total arthroplasty.