The European Integrated Project EU-DEEP, which deals with the integration of Distributed Energy Resources in the transmission and distribution system, proposes technical, operational and regulatory analyses, developed in a coordinated way, as the best possible starting conditions to this integration. Distributed Energy Resources (DER) are already operating in Europe. However, the trend towards smaller size combined heat and power generation, and renewable generation connected to low voltage networks, introduces new challenges concerning the trade-off between the potential benefits of DER in general and the detrimental effects they may have on distribution and transmission systems. Comprehensive analyses of the electrical power system operation in normal and in emergency conditions are necessary in order to classify these effects of DER and for suggesting possible practical solutions. This means solutions which can be implemented efficiently, balancing adequately the additional investments and the cost of degradation in terms of reliability and quality. The paper briefly summarises the general methodology that has been originally developed within the EU-DEEP project. It also presents elements that permit to address grid-integration issues for DER, their capacity to operate satisfactorily in the distribution system with high penetration ratios.