Electricity generation and transmission activities have the potential to cause a wide range of environmental and social impacts, which are for the most part not integrated into the pricing system (neither are in the cost evaluation of projects); hence they are referred to as “externalities” or “external costs”. External costs should be internalised (by means of taxes, subsidies, or other economic instruments) in order to rebalance the social and environmental dimensions with the purely economic one, leading to greater market efficiency and environmental sustainability. The evaluation of the external costs could also be of great help in cost-benefit analysis, allowing the “negative” impacts being compared with the social (and environmental) benefits of transmission lines. The first step is, therefore, the quantification of externalities. The external costs methodology, applied to the energy system, has been developed in the frame of the European Project ExternE (started in 1991, www.externe.info), and further improved and refined in the frame of the recent project NEEDS (www.needs- project.org), both focused on the quantification of power generation externalities, with particular emphasis on the adverse effects of airborne pollutants, while the external costs of electricity transport have been somehow neglected. The problem appears to be a challenging one, because –on one side- it is highly site dependent, and –on the other side- the impacts to be evaluated often involve goods that do not have a “market” (e.g. the landscape, the biodiversity etc..). The most important impacts are concerned with: landscape, land use and territory, electromagnetic fields (EMF), noise, ecosystems and network power losses. In this paper, we propose a comprehensive methodology for quantifying the externalities of electricity transportation, with a special focus on High Voltage Over Head Lines. The methodology is based on the well-known “Impact Pathway Approach” established by the ExternE Project. The sum of all this kind of externalities showed to be highly variable, depending on the line and on its location, ranging from small values , to more than 1.5 M€ per year per km.