Frequency is currently the best measurable quantity that contains information about the overall balance between generation and consumption. For this reason, an efficient and tight tracking of the frequency allows a secure operation of power systems. The chapter revisits the concept of frequency control and provide a comprehensive overview of it. In particular, the chapter presents various terms and concepts related to frequency control that are often utilized with different meanings by different TSOs. Finally, the chapter discusses how to navigate through the current plethora of terms surrounding the topic of frequency control.