This chapter examines power system strength, its definition, and calculation methods. It covers the following topics: classification and external characteristics of non-synchronous-machine sources; their equivalent circuits during normal and fault states; classical short circuit ratio expressions (capacity and impedance short-circuit ratios); voltage stiffness index definition and its comparison with impedance short circuit ratio; Thevenin equivalent impedance calculation at any grid node; voltage stiffness and impedance short circuit ratio calculations for typical non-synchronous-machine generation stations; key factors affecting voltage stiffness and impedance short-circuit ratio; applicability of capacity and impedance short-circuit ratios in new energy power systems; controller modifications for enhanced voltage support; grid equivalent reduction methods for new energy bases based on constant voltage support strength; multi-infeed voltage stiffness and impedance short circuit ratio concepts and applications; multi-infeed effective short circuit ratio applicability in new energy power systems; implementation of inertia support and primary frequency regulation for non-synchronous-machine sources; and methods for calculating their inertia support strength and primary frequency regulation capabilities.