Until relatively recently, the National Energy System Operator (NESO) and Distribution System Operator (DSO) performed distinct, and largely independent roles. Both organisations could consider the other's networks as being constraint free and having ready ability to access DER for their own needs. Increasing volumes of services from DER for both transmission and distribution system needs mean that the need for co-ordination between system operators is becoming increasingly imperative. This chapter considers how the underpinning rules around conflict management have been determined and the need for continued development of primacy rules.