Valgus-producing osteotomy of the proximal tibia is a well-established treatment for medial femorotibial osteoarthritis in the varus knee. The ideal patient is active, under 55 years of age, has a stable varus deformity of less than 10 degrees, a good bone stock, and an osteoarthritis stage that is restricted to the medial compartment of the knee. Coventry reported a failure rate in proximal tibial osteotomy to be significantly higher when the postoperative alignment was less than 8 degrees of anatomical valgus. Hernigou noted better clinical long-term results in cases with a precise correction of malalignment. There are different preoperative planning methods varying between simple estimates of correction angles and specific radiographic planning tools. The reproducibility of operative outcome with regard to a predictable anatomic alignment and functional recovery must have high priority. This chapter deals with different preoperative planning methods to improve the reliability of surgical results after tibial osteotomy.