The chapter explains the essence of the ‘weight design’ concept and applies this concept to estimate an airplane’s gross weight in a third iteration, including the fuel, power plant, and airframe weights. The chapter also shows how an aircraft’s gross mass may be recalculated if some changes to the baseline design are introduced. The chapter discusses the concept of mass budgeting and control and how these approaches can be realized in engineering practice. Finally, the chapter explains the moment of inertia of aircraft and its prediction.