The steady increase of renewable energy (RE) together with the shift away from fossil fuels has led to the withdrawal of conventional synchronous power plants from operation. Hence, the onus is now on RE to provide the system services. The procurement of balancing services and activation of balancing energy bids, frequency control and balancing requires co-operation between all parties involved, i.e., transmission system operators (TSOs), distribution system operators (DSOs) and distributed energy resources (DER)/aggregators. This chapter examines the interaction and coordination between transmission system operators (TSOs) and distribution system operators (DSOs) to maximize the benefits of renewables, storage and demand-side resources while maintaining overall system adequacy and security. Expectations with respect to the availability of real and reactive power, requirements for reserve capacity, system restoration and planning and operations are among the many changes considered. This chapter focuses on two critical aspects of the TSO–DSO relationship: Frequency management and balancing are observed as being increasingly managed through a coordinated approach between transmission-level and distribution-level entities. Operating reserve capabilities of DERs deserve special attention as they present significant reliability issues relating to the provision of bulk system services. When distribution-connected resources are activated by the TSO, impacts on neighboring TSOs who are responsible for ensuring overall balance within their areas of responsibility must be taken into consideration.