Book excerpt: Mechanotherapy in Orthodontics: Principle of Design, the volume II of this book series, presents the mechanical principles instructing a precise and personalized mechanotherapy plan. Rather than analyzing the different appliances or techniques used in the daily Orthodontics and Orthopedics treatment, we introduce important concepts such as, anchor versus target unit, sequence of mechanotherapy, semi-restricted and free-object designs, material behavior as source of forces, and friction in orthodontics.Deep understanding of these concepts equips the clinician to develop a personalized Mechanotherapy to efficiently treat any malocclusions despite their complexity.