This chapter explores the impact of inverter-based resources (IBRs) on the oscillatory characteristics of bulk electric systems. It focuses primarily on the electromechanical modes of oscillation and low-frequency control modes that can be studied using phasor-domain tools. Two inverter control models approved for use in transmission planning studies, one grid-following and one grid-forming, are investigated. We use linearization and eigenanalysis to determine the sensitivity of local and inter-area modes of oscillation to increases in the penetration of IBRs. Discrete effects arising from synchronous machines being displaced by renewable generation are also explored. To illustrate the main concepts, analysis and simulation results are presented for a two-area test system augmented with solar photovoltaic generation. All analysis is performed in MATLAB using the Power and Energy Storage Systems Toolbox (PSTess). This package is an extension of PST designed for studying systems with a mix of conventional and inverter-based resources.