Insights on designing a sustainable 6G system as a multi-functional platform that delivers services beyond communication 6G to Build a Sustainable Future provides a summary of the research conducted in the European 6G Flagship project Hexa-X-II towards the sixth generation (6G) mobile networks, with additional input from other smart networks and services joint undertaking (SNS-JU) projects, such as 6G-DISAC, 6G-MUSICAL, 6G-NTN, 6G-SHINE, Deterministic 6G, RIGOROUS, ROBUST-6G, TERRAMETA and TERA6G. The book explores the motivation, values, and needs of 6G, with a strong emphasis on environmental, economic, and social sustainability. To address these needs, the 6G system will be designed as a platform, providing services including and beyond communication; the book provides an end-to-end (E2E) blueprint of this system. The book also outlines the potential paths from the project results towards standardization and further towards introduction to the markets. Topics discussed include: Design principles, requirements, value, validation and blueprints for inclusive, trustworthy, and environmentally sustainable 6G platforms 6G transceiver and radio design, covering architecture and deployment, radio link modeling, transmission schemes, and signal processing hardware 6G intelligence through AI-native architecture, smart network management, and intent-based management Architectural enablers including flexible networks, dependable networking, beyond communication services architecture, and radio protocols E2E security concepts, covering security enablers All this from Hexa-X-II as well as other SNS-JU projects 6G to Build a Sustainable Future is an essential, up-to-date reference for wireless researchers, network planners, technology analysts, technology marketers, R&D engineers, application developers, spectrum regulators, and students.