The Digital Twin book is about harnessing the power of technology, business practices, and the digital infrastructure to make revolutionary improvements for the benefit of society. Ninety experts from around the world contributed to summarize four decades of digital advances and successes, and to define the Digital Twin's potential for the decades ahead. The book describes how Digital Twins will play a key role in specific applications and across important sectors of the global economy, making it a must-read for executives, policymakers, technical leaders, researchers, and students alike.  The book consists of thirty-eight chapters that cover Digital Twin concepts, supporting technologies, practices, and specific implementation strategies for various production and service sectors.  Digital Twins are about creating faster, less expensive, and error-free manufacturing, products, processes, and services. This includes engineering of systems for energy, communications, construction, transportation, and food processing. It also covers solutions for making human existence better and more enjoyable through the life sciences, smart cities, and artistic creations. The Digital Twin's functionality addresses the entire lifecycle of products and services. Importantly, the book describes the journey required for businesses and public organizations to embrace Digital Twins as part of their tool kit.  The Digital Twin is the ideal starting point for teaching and research in all application domains.