The ever increasing demand for communication bandwidth has led to spectrum scarcity. We need to manage spectrum as a scare resource. Better utilization of spectrum is the key requirement for the radio communication systems. The inefficient usage of the existing spectrum can be improved through opportunistic access to the licensed bands without interfering with the primary users. This introduces the concept of dynamic spectrum access. Cognitive radio is must for providing quality of service in dynamic spectrum access. To deal with the cognition at physical layer, upper layers are also modified to support cognition and adaptation which make a network cognitive network. This book chapter describes the technical challenges and approaches for cognitive radio networks. Spectrum access models from regulatory point of view are presented. It also explains the challenges and solutions from literature for physical, MAC and network layer of cognitive networks. In the end of the chapter, real hope for designing protocols for such networks, i.e. cross layer protocol design is discussed.
