This book is about the engineering of Digital Twins (DTs) of cyber-physical systems (CPSs). It goes behind the glossy image of DTs to help researchers and advanced professionals to ask and answer the fundamental questions underpinning the development of a DT. What are the foundational concepts of the DT? How do different engineering disciplines interact in creating a DT? How should the physical and digital worlds be connected, and how do the imperfections and faults inherent in both worlds affect the DT's qualities? How can we use a DT to support decisions, and how do we maintain it through life? To this end, the book is structured in five parts: "Foundations" introduces the DT concept, the potential benefits of DTs seen from a business perspective, and foundations for DT engineering. "Models and Data" presents the range of models and data that form the core assets of DTs for CPSs. It covers ways in which models can be produced and calibrated, and considers how data is derived from a CPS and communicated to its DT. Next, "Services for Digital Twins" details some of the main services that a DT provides by building on the assets of models and data, including visualisation, fault detection and diagnosis and support for decision-making. "Realising Digital Twins" then covers the realisation of DTs, including a platform allowing engineers to construct DTs from reusable components. Case studies in food production, robotics and marine engineering are presented using a systematic framework that aligns with the DT engineering concepts introduced in the earlier parts of the book. Eventually, "Advanced Topics in Digital Twins" introduces advanced topics in delivering dependable DT-enabled systems, focusing on security and privacy, the capacity for autonomy, and a range of open research topics. This book aims at researchers in DT technology and design, including advanced (master and doctoral) students, as well as engineering practitioners aiming to develop DTs. The most common techniques described in the main text will be accessible via open-source projects, including further DT examples, exercises and solutions, as well as pointers to emerging standards, frameworks and platforms. Classroom materials, exercises and solutions are available to lecturers through a dedicated Web site.