The electricity system is undergoing deep changes as a consequence of ongoing evolution in policies to reduce greenhouse gas emissions, develop renewable sources and improve energy efficiency. The development of energy infrastructure is also part of this perspective. Many people expect that the transition from traditional grids to the so called Smart Grids will make it possible to tackle the complexities arising from decentralized and non programmable energy production, the massive electrification of consumption, and the increasing participation of prosumers in the energy market. Nevertheless, the costs and benefits of SGs remain a debated topic among experts, operators and national regulators. It is therefore necessary to develop robust tools to assess investments in this type of infrastructure, in order to understand the possible effects of such interventions both on the electricity system and the community. These topics are the subject of this article, along with a description of a new methodology for Cost Benefit Analysis for Smart Grids in the distribution grid.