Nowadays the transport of electric energy through overhead power lines is a critical infrastructure and hence adequate maintenance is paramount. The lines, support poles, insulators, and all the associated equipments for a complex system with highly specialized maintenance operations. This paper focus on the maintenance of the lines themselves, namely in checking the physical state of the conducting material. Verifying the conditions of the lines while in operation, i.e., while transporting energy, is a delicate operation involving working under strong electromagnetic fields, at height, under precarious equilibrium and meteorological conditions. In addition, the length of the lines is commonly in the range of thousands of km, thus a regular maintenance represents a considerable time, effort, and cost. Originally, maintenance was performed with workers verifying, in loco, the condition of the line. The development of manned and unmanned vehicles capable of carrying sensing tools to detect anomalies brought new possibilities and contributed to improving the safety and health conditions of the workers. The paper reviews the methods currently being used by the industry to verify the condition of the lines and discusses the effectiveness of robotics technologies.