This chapter covers all fundamental concepts, design theory, and formulas for mechanical design without any examples for theoretical hand calculation because all calculations will be done by FEA (Finite Element Analysis) simulation. First, it uses the five-phase design process to discuss and explain an engineering design process. Second, it discusses and explains the definitions of stress and strain; it provides typical stress formulas such as normal stress of a bar, direct shear stress, shear stress of a shaft under torsion, bending stress, and shear stress of a beam under a shear force, as well as it explores principal stresses and maximum shear stresses of a general state of stress. Third, selections of materials and manufacturing processes are explained and discussed. Fourth, it explains the definition of free dimension and mating dimension and shows how to determine the dimension tolerance for free dimensions and mating dimensions. Finally, it explains the classification of materials, material mechanical properties, definitions of mechanical failures, typical failure modes, and typical failure theories for ductile and brittle materials. Therefore, this chapter is a concise summary of mechanical design. It prepares us to be ready for conducting mechanical design while stress, strain, and factors of safety are determined by FEA simulation.