Distributed Energy Resources (DER) are decentralised small-scale power generation sources located near end-user consumers that supply part or all of their electricity demand. DER are increasingly seen as key to the development of a sustainable low-carbon power system. DER technologies offer opportunities for the development of renewable energy but also pose challenges for their integration in the power system. Opportunities have been created by the reducing cost of DER technologies, in particular solar photovoltaic (PV) and by the possibility of using DER for system flexibility. Moreover, DER represent an important vector for the diffusion of electricity in developing countries. One of the challenges is the difficulty of managing and designing the electrical grid, which was not designed to have several small-scale power generators connected to the distribution grid. Many DER will still need policy support in the coming years, which must be rationally designed. A large deployment of DER requires the development of a new framework for the power sector in all areas.