Orthodontic biomechanics is the foundation of orthodontic treatment. It is extremely important to fully understand orthodontic biomechanics before the clinician /orthodontist may utilize specific orthodontic appliance. This chapter highlights basic principles of biomechanics of tooth movement with emphasis on forces, moment, moment to force ratio and its importance in achieving different types of tooth movement. Also, this chapter provides detailed explanation of different types of tooth movement and relativity of moment to force ratio as well as center of rotation approximate location in each type of tooth movement. The application of these concepts with clear aligners is somehow different from the way fixed orthodontic appliance has been used. Explanation of these differences is presented.