This work focuses on the facilitation of aggregator companies for the provision of flexibility services to the electricity system by using large numbers of dispersed and distributed resources of small capacities. This initiative by TenneT TSO and Utrecht University is about identifying potential barriers and proposing solutions and actions for enabling flexibility in power systems operations, from scheduling and operations, to verification and settlement, within the current systems of programme responsibility and imbalance settlement in the Netherlands. The project activities address the feasibility of new concepts for the provision of flexibility in the energy system by focusing on the market integration aspects. First, the procedures and requirements regarding the current systems of programme (balance) responsibility and imbalance settlement in the Netherlands were analysed, i.e., operational planning (nomination and scheduling of exchange, bidding for regulating, reserve and incident reserve power), operations (request for upward and/or downward power, and dispatch), and settlement (verification and financial settlement). Subsequently, different options for contributing to system balancing through the bid price ladder (active contribution), voluntary response (passive contribution), emergency power (contracted emergency capacity) and exchange of flexibility, from an aggregation of distributed resources, between market parties were analysed. Finally, regulatory, institutional and technical barriers for developing flexibility mechanisms through aggregator companies were identified, and potential solutions were explored for overcoming identified barriers and complying with technical requirements. This report provides a state-of-the-art review of the Dutch electricity market and highlights all relevant issues for advancing the market integration of aggregator companies within the Dutch system, and in line with the new European grid codes. Historical and emerging cases of demand response implementations through ...