Overhead lines, underground cables, and gas-insulated lines (GIL) are collectively referred to as transmission lines, as they all serve the same fundamental purpose: to reliably and efficiently transfer electrical energy between network nodes. The chapter begins by examining how the different technologies affect their electrical paramters, such as ampacity, demand on reactive power and transmission losses. It then explores the behaviour of the electric and magnetic fields in the vicinity of each transmission type. Based on statistical data on outages and damage, reliability parameters are derived using mathematical models, such as the Markov method. These and other operational considerations, e.g. neutral point treatment, are illustrated through practical examples. The chapter then introduces tools to calculate the main operating states relevant for planning and operation. Several step-by-step calculation examples are presented for the different line types with results visualized using phasor diagrams. The most important characteristics of the line types are compared, both in alternating (AC) and direct current (DC) systems. A dedicated section discusses the construction costs of these line types.