This book advocates for the adoption of digital twins as technological artifacts of smart factories to support decision-making by different roles in a manufacturing company. Digital twins of manufacturing systems, in fact, support decisions in the various activities required for their design, management, and improvement. Furthermore, they represent a promising prospect for a future characterized by human-centric, sustainable and circular manufacturing. Given the rapid progress observed in recent years, which has led to conceptual fragmentation, heterogeneous implementations, and divergent interpretations, a forward-looking vision is therefore offered on the role of digital twins for the future of manufacturing. The book is structured into three parts, which reflect expectations regarding the theorization and practice of digital twins. The first part illustrates the conceptual basis, identifying the key characteristics that differentiate digital twins from digital models, while also analyzing the virtualization of decision‑making processes. The second part aims to delve deeper into the topic of digital twins from both an operational and architectural perspective, discussing their role in collaborative decision-making, as well as the architectures and frameworks, methodologies, and technologies for the development and implementation of digital twins in the context of cyber‑physical integration of manufacturing. The third and last part aims to illustrate how digital twins apply to the entire lifecycle of a manufacturing system, supporting design and commissioning, production management and control, reconfiguration and performance management, and ultimately extending to sustainability, circularity, and human-centricity. This part concludes with an overview of future research directions. Overall, through its forward-looking vision, the book’s overarching goal is to inspire innovation relevant to manufacturing practice, share knowledge to stimulate research advances, and contribute to future human and technological development.