While fifth generation (5G) systems are being deployed worldwide from 2020, there is already a major interest in beyond 5G i.e. Sixth generation of wireless communication systems. Hence, it is envisioned that 6G networks will provide new communication services, network capacity, and ultra-low latency communications. In this book, first, the Vision and requirement of 6G networks is discussed. Then, the potential key enabling technologies for 6G networks are presented. In particular, we will focus on Terahertz communication technologies, intelligent surfaces, Visible light communications and Index modulation. Finally, several challenges for future 6G networks and potential future directions are presented