A rising reliance on inverter-based resources diminishes synchronous inertia, thus intensifying frequency fluctuations in power systems. The measurement of Rate of Change of Frequency (RoCoF) is crucial for ensuring stability amid these rapid dynamics. This chapter illustrates how reduced inertia accelerates frequency drifts, heightening the need for precise, timely RoCoF sensing. It outlines phasor measurement unit’s estimation methodologies and highlights the impact of phase steps, harmonics, and noise on RoCoF measurements. Practical applications like fast frequency response, under-frequency load shedding, and loss-of-mains protection demonstrate the centrality of robust RoCoF data. Even minor inaccuracies risk unwanted disconnections or insufficient corrective measures, jeopardizing low-inertia systems. The discussion concludes by linking refined RoCoF detection with overall grid reliability, emphasizing how accurate measurement underpins secure operation in networks dominated by inverter-interfaced renewable generation.