Preoperative planning is routinely recommended prior to total knee arthroplasty (TKA). We introduce a methodology for planning TKA based on mechanical alignment. A methodology for planning total knee arthroplasty was discussed among experienced knee surgeons. A rational methodology for planning TKA was stablished and it was setted to an application for mobile devices. It has proved to be useful and revealed accuracy compared to the manual form of preoperative planning. It was able to reduce planning time by more than a half and it was still reliable in measuring the anatomical-mechanical femoral angle (MAFÂ). This chapter introduces a TKA planning method based on mechanical alignment and GAP balancing principles. Kinematic alignment and strategies for soft-tissue balancing in special situations are cited as well.