While the 5th Generation (5G) system is being widely deployed across the globe, the information and communication technology (ICT) and academic communities have started looking into the next generation of mobile systems, namely 6G. Although 5G has improved on many relevant key performance indicators (KPIs) (e.g., peak data rate, latency, reliability), its design is still restricted to limits that might prevent some novel use cases to become a reality. Key examples are haptic communication, holographic telepresence, and digital twins, which all require a combination of high data rates and low latency. Furthermore, the sustainability of the mobile system is becoming a major concern, as the energy consumption of the network is increasing with the deployment of 5G. This book provides a comprehensive overview of the current state of the art in 6G research, identifying the key challenges and enablers for the next generation of mobile systems. It also proposes a novel architectural design for 6G, which is based on the concept of sustainable and trustworthy 6G. The book is the result of the collaboration of a large number of partners from the Hexa-X project, the European flagship project on 6G.