ers to the technology where a UE is simultaneously connected to two different radio access network nodes, one providing NR access and the other one providing either LTE or NR access. This allows a UE with multiple RX/TX capa- bilities to increase the data transfer rate thanks to simultaneous reception and transmission over the two different radio links. Unlike some other RAN architectures described in the book, MR-DC is not just a choice of RAN architecture, as it has substantial UE impacts. Therefore, even though the primary focus of the book is RAN architecture, in the present chapter we also describe certain UE-related aspects, which are important to fully under- stand how MR-DC works. From the network architecture point of view, one of the two network nodes serving the UE acts as the Master Node (MN), that is, the access node that terminates the control-plane connection with the core network and provides primary radio resources via one or more cells - the Master Cell Group (MCG) - under its control. The other node acts as a Secondary Node (SN), that is, an access node with no control-plane connection to the core network (and with or without a user-plane connection to the core network) providing additional radio resources to the UE via one or more additional cells, that is, the Secondary Cell Group (SCG). 5G MR-DC is the evolution of LTE Dual Connectivity (DC) defined as early as Release-12. Even though LTE DC has not been widely deployed, it heavily influenced a number of tech- nologies defined in 3GPP, ranging from LTE-WLAN Aggregation (LWA) to 5G MR-DC. The MN and SN are connected together via a network interface that allows the exchange of control-plane and user-plane information. The MN and the SN are logical nodes, which may be deployed in a single physical node or separate ones, as is the case for all other net- work architecture options described in the book. It is generally assumed that the MN node is a macro base station providing coverage, whereas the SN node is a small cell providing additional cap