This paper presents the design foundations of an agile resource management (RM) framework for the fifth generation (5G) radio access network (RAN) by describing essential building blocks (BBs). Overall, the presented RM framework provides holistic RM solutions that consider and exploit the novel aspects of 5G systems, such as, diverse service requirements, co-existence of multiple air interface (AI) variants (AIVs) in the overall AI, dynamic radio topologies, and novel communication modes. The proposed framework encompasses the following key BBs: Advanced interference management techniques for dense and dynamic deployments; dynamic traffic steering mechanisms that aim to attain the optimum mapping of 5G services to any available resources when and where needed by considering the peculiarities of different AIVs; tight interworking between novel 5G AIVs and evolved legacy AIVs such as Longterm Evolution (LTE); sharing of a common RAN by multiple network slices; RAN moderation techniques to attain energy efficiency gains; and functional extensions in the device measurement context to enable all the above new functionalities. The BBs are analyzed from the perspectives of key findings and RAN design implications.