High tibial osteotomy is one of the knee preservation surgeries for varus malaligned young arthritic knee. Prerequisite for high tibial osteotomy is reported in younger patient, less than 55 years of age, BMI of around 30, metaphyseal tibia varus, grade III to grade IV osteoarthritis of the medial compartment of the knee, varus up to 13–14° with normal lateral compartment and mild to moderate patellofemoral osteoarthritis. The two most common methods are Miniaci and Dugdale–Noyes methods to calculate varus deformity for preoperative planning. These methods are based on mechanical axis, medial proximal tibial angle, lateral distal femoral angle and joint line congruency. The reproducibility of surgical outcome with regard to a predictable mechanical alignment and functional recovery is important. This chapter deals with preoperative planning methods and their related complexities to improve the quality of surgical results after High tibial osteotomy.