The spread of inverter-based generation technologies, such as wind turbines and solar photovoltaics, is challenging the traditional assumptions around system inertia frequency response of conventional generation. Conventional synchronous generation has mechanical inertia and spins at a constant, or synchronous, speed, which limits the rate of frequency drop if there is a loss of generation, providing time for turbine governors to intervene. Inverter-connected, non-synchronous generation sources, such as wind turbines or solar photovoltaics, do not possess “inertial” response or primary frequency response. New developments in technology and markets are making it possible for non-synchronous systems to hold energy sources in reserve that respond like, or emulate, conventional inertial and primary frequency response. Work remains to fully understand how emulated inertia, sometimes called synthetic inertia, might provide an additional, effective tool for grid stability.