Overhead lines are the backbone of the electrical power transmission. Contrary to the distributions networks, the transmission system consists only in exceptional cases of longer cable lines. Typical exceptions are connections of cavern power plants, approaches to airports or bird sanctuaries and lines in urban centres. In the majority of cases, an overhead line is the most economic and practicable solution for the energy transmission. In tourism regions, an overhead line will be seen as impairment of nature or landscape and so the approval chain and procedure is in most countries long-winded and circumstantial. At the other hand, the energy consumption in Europe is growing and the volatility of transmitted power is also increasing during the last decade caused by the opening of the electric energy market. This opening process leads to a stopping of the enlargement of the interoperation network and to a minimisation of the maintenance of existing lines. Today the network operates more often at the limit of the equipment and the small and large-areas disturbances and blackouts are increasing. The operators of transmission lines are forced to ensure the electrical power supply and so they have to improve the reliability of the network. One solution is to monitor the critical (heavy loaded) overhead lines. For example, with the knowledge of the thermal condition, the risk of unexpected outages can be reduced.Today several monitoring systems are available on the market. They differ in the principle and techniques of the condition evaluation. The three most interesting output variables are the line temperature, the capable transmission power and the actual sag of the investigated section. In this paper an overview of existing overhead line monitoring system and also an outline over the usage and benefit for the application will be given. Thermal monitoring is one technique to improve the reliability of the network and for increasing or optimising the capable transmission power.