This chapter discusses the recent evolution of power systems, highlighting the need for a comprehensive paradigm shift, particularly in the approach to frequency and rate of change of frequency (RoCoF) estimation. The high penetration of renewable energy sources, typically interfaced via power converters, introduces fast dynamics that are not compensated by sufficient levels of inertia. Therefore, the system frequency and its first-order time derivative are no longer static state variables, independent of time and location. It is thus necessary to develop measurement infrastructures capable of accurately measuring these parameters in relevant grid nodes, using this information to undertake the most suitable control actions to keep safe and stable operations. The chapter begins with a brief overview of the regulatory framework for frequency and RoCoF estimation. It then introduces the problem of comparing and aggregating measurements coming from different sources and/or algorithms. In this context, the chapter also discusses the uncertainty contributions inherent in the definition of the measurand and proposes alternative approaches for the characterization of power system signals in the presence of fast dynamics. Finally, two real-world PMU-based applications for event detection and system control are discussed.