This chapter offers a foundational overview of key concepts in electrical engineering that are essential for understanding power transmission systems. It addresses fundamental topics such as the transformer principle, the root mean square value (RMS), and the various forms of electrical power-active, reactive, and apparent-which together provide the basis for analyzing power flow and efficiency. Following this, the chapter presents the historical development of power transmission infrastructure, focusing on both overhead lines and underground cables. This evolution is outlined chronologically, beginning in the late nineteenth century and extending to contemporary advancements. Particular attention is given to the initial debate between alternating current (AC) and direct current (DC) systems, as well as to the subsequent development of standardized voltage levels that facilitated the emergence of modern supply networks and interconnected grid structures. The chapter concludes with a discussion of key planning principles, including the (n-1) criterion, and underscores the critical role of system frequency as a fundamental parameter for ensuring the stability and security of electrical power systems.