Underground cables are used in place of overhead lines where the latter cannot be technically implemented—for example, when crossing bodies of water or nature reserves, or when supplying islands and offshore platforms. In addition, underground cables are indispensable in densely populated urban areas with high energy demands, particularly across much of the industrialized world. This chapter introduces the fundamentals of cable construction, which—beyond conductor material and cross-sectional area—is largely defined by the type of insulation and the protective measures against contact and mechanical damage. It begins with key characteristics of the two most commonly used conductor materials, copper and aluminum, along with an explanation of the skin effect and proximity effect. Next, the concepts of service life and operating capacity are discussed, followed by detailed descriptions of the main high and extra-high voltage cable types: mass-impregnated cables, oil-filled and gas pressure cables, extruded insulation cables (including submarine cables), and gas-insulated transmission lines (GIL). Subsequent sections address the specific design differences between alternating current (AC) and direct current (DC) applications. Under the topic of cable accessories, joints and terminations are covered, including their construction as plug-in types and the standard grounding techniques—such as cross bonding. Various installation methods are then presented and compared in terms of their advantages and disadvantages, with particular attention given to cable transport and pulling procedures, including relevant calculation methods. The chapter also explores cable and soil heating, along with possible cooling techniques. The chapter concludes with an overview of commissioning tests, monitoring systems, and fault detection methods.