This chapter provides an overview of the definitions and estimation techniques of the fundamental frequency of ac grids. The chapter explains the link between frequency variations and power imbalances and why frequency, similarly to bus voltage magnitude, is different from bus to bus in transient conditions. The chapter also discusses the conceptual challenges of defining the frequency of non-stationary ac voltages and describes relevant frequency estimation methods and their features and limitations. Finally, the chapter defines the RoCoF and its role in power system operation and protection.