Future distribution networks are expected to host high concentrations of distributed energy resources (DER) which include distributed generation, renewable energy sources, local storage systems and flexible loads. This book introduces a DER aggregation system (DERAS) that incorporates DER in the power system in an efficient and cost-effective manner. DERAS is developed based on the concept of the virtual power plant to facilitate the collective participation of the DER owners in the wholesale electricity market. The commercial interests of its stakeholders are incorporated in a trade subsystem that employs artificial neural networks to forecast the contribution of the aggregated DER population. Moreover, through the balancing and network support subsystems DERAS helps system and network operators to postpone or prevent excessive expenditures needed for network expansions or reinforcements. This book involves Systems Engineering to develop a scalable DERAS that fits in either competitively unbundled or traditionally consolidated electricity markets. Moreover, it presents an active network planning method that facilitates the development of Smart Grids.