The Long-Term Evolution (LTE) radio protocols were primarily designed for the provision of PS services through a flat architecture. This chapter describes the 5G radio protocols, with a focus on the modifications and enhancements brought to the LTE baseline. The Service Data Adaptation Protocol layer handles the mapping of QoS flows from a PDU session onto Data Radio Bearers (DRBs). The Packet Data Convergence Protocol layer handles DRBs and guarantees in-order delivery without duplicates to the upper layers, performs header compression when required, and is the layer responsible for enforcing security in the RAN through ciphering and integrity protection. The main tasks of the Radio Link Control sublayer are segmentation and error correction. The Medium Access Control layer is the lowest sublayer of Layer 2 and is responsible for interfacing with the physical layer.