SSince the big success of Artificial Intelligence (AI) in various fields has inspired in wireless communications and networks, 6G systems will be powered by AI. The 6G systems have a great potential for improving the performances and system optimization by AI. They already started to affect and enhance them each other. We expect to redefine the landscape of wireless communications and networks. Thus, many research groups in wireless communications and networks are trying to combine wireless technologies and AI and bring a synergy from them. This topic becomes one of key technical trends toward 6G. They have lots of technical and non-technical challenges. The technical issues cover how to satisfy the high-level system requirements in terms of throughput, latency, energy efficiency and also improve the sustainability of 6G networks. The non-technical issues contain how to develop 6G use cases and business models and also meet the regulation and ethical guidelines. In this chapter, we discuss the fundamentals of wireless communications and networks and provide a comprehensive study of key techniques in wireless communications and networks.