This chapter explores the definition and analysis methods of power system resonance stability. It covers the definition, physical mechanisms, and properties of resonance stability. The chapter examines the differences between “wideband resonance” and “wideband oscillation,” and presents the theoretical foundation of s-domain nodal admittance matrix method. It discusses factors that determine resonant modal damping and the basic characteristics of weakly damped systems. The chapter then details the overall approach of the s-domain nodal admittance matrix method, including the structure of resonant modes in damping-free systems and calculation methods for their resonance frequencies. It also explores the significance and calculation methods of nodal voltage mode shape and node participation factors corresponding to resonant modes. Finally, the chapter addresses sensitivity analysis of damping values for resonant modes and discusses why analyzing resonance stability based on sequence network models is inappropriate.