In electric power systems, the balance of generation and consumption is ensured through frequency control. This chapter aims at introducing the concept of “frequency control strength” as a novel approach to understanding how different real-world power systems compare to each other in terms of the effectiveness and performance of system-wide frequency control. The chapter presents a comprehensive comparison, based on measurement data, of the frequency control strength of four real-world, renewable-based, synchronous island power systems, namely GB, AIPS, mainland Australia, and Tasmania. The strength is evaluated by means of different short- and long-term frequency quality metrics.