A key concept and requirement for an operational and effective deployment of a large number of Distributed Energy Resources (DER) to enable the provision of flexibility and grid services is the aggregation of DER. A DER Management System (DERMS) provides this aggregation function, as well as the functions required to enable grid services. This guide provides a functional specification for a DERMS and describes the grid services that aggregated DER can provide the distribution and transmission systems. It addresses implementation issues and discusses the interoperability requirements of a DERMS with its environment, including the transmission and distribution systems, and the communication and information infrastructure of modern grids.