The mechanical axis of the lower extremity is determined by drawing a line from the center of the femoral head to the center of the ankle and passing through the center of the knee joint. The terms valgus and varus knee refer to mechanical axis deformities and they have been reported as major risks of knee osteoarthritis. The concept of osteotomy is to correct limb deformity in order to reduce pain, delay the degenerative process and the joint replacement. The basic principle of the procedure is to induce a surgical transection of a bone to allow realignment and a consequent transfer of weight bearing from a damaged area to an undamaged area of joint surface. Main indication is the correction of varus malalignment by performing a high tibial osteotomy (HTO) in medial unicompartmental osteoarthritis of the knee. Knee osteotomy is most effective in young patients (<50 years) with pain only on one side of the knee and no retropatellar pain. For most patients, osteotomy is successful in relieving pain and delaying the progression of arthritis in the knee. It can allow a younger patient to a more active lifestyle for many years.