The penetration of Distributed Energy Resources DER within the Distribution Network DN increases worldwide. This increase of local generation has led to various changes in the philosophy of DN operation. The changing nature (higher participation of power electronic components) of connected system loads and distributed generation DG is creating new challenges for the operation of transmission and distribution networks and DG units will be requested to provide more ancillary services. When DER reach a certain level of penetration, they have an impact on the overall steady state and dynamic performance of the bulk electric power system. Moreover, the increased power injections at the DN can cause overvoltage problems and reverse power flows that may affect the protection settings within the distribution system. A DN with high penetration of DERs responds dynamically to system disturbances and therefore affects overall power system stability and dynamic behavior. DER, when properly controlled can provide support to the upstream network by supporting the reactive power needs of the loads (at the distribution level) and flow reduction in case of congestions by supplying the demand at the distribution level. Control can be basically exercised by exploiting the power electronic converters used to interconnect most types of DER. The provision of support to transmission network TN operation by DER is already required by operating codes in some countries. This technical report is organized as follows, Section 2 presents the main impacts of high DER, particularly photovoltaic generation, penetration at the distribution grid on the operation of the bulk power system. Section 3 provides ways to control DER in order to solve these issues. Section 4 presents several use cases of the impact of DER at the bulk system and their ability to support transmissions network stability. Section 5 presents aggregate and equivalent DER models that can be used for steady state and dynamic analysis studies and a method for obtaining dynamic equivalents of active DNs, in order to assess their impact on the overall power system dynamic behavior. Section 6 presents a discussion on the modeling and control of microgrids in supporting DER integration and providing network services. Section 7 highlights recommended extensions of Regulations and Network Codes in order to improve cooperation of TN and DN, and Section 8 summarizes the main conclusions of the report.