This book investigates strategies for integrating sustainability objectives into the design and operation of Digital Systems. Instead of providing a pure academic introduction, the book will take a hands-on approach facilitating an understanding of practical implications of the European Green Deal and specific needs to be fulfilled by commercial products to maintain continued access to the European Union Single Market. Building upon the European Union's Green Deal, which prioritizes sustainable development, we explore a range of relevant concepts and initiatives. We begin by examining how high-level political aspirations translate into concrete actions, focusing on regulations such as Ecodesign legislation that influence market access and drive the development of sustainable technologies. We then delve into the technical aspects of sustainable digital systems, including considerations for Digital Architecture, the interplay between Cybersecurity and Sustainability, and the ethical dimensions of sustainable digital development. Furthermore, we explore real-world applications through case studies and expert interviews, examining how various sectors and stakeholders are adapting their practices to meet sustainability goals. This includes an in-depth analysis of emerging technologies such as 6th Generation Cellular Networks, AI for Wireless Communications, and the Internet of Things, with a focus on their potential to contribute to sustainable development. Finally, we compare the European approach with sustainability initiatives in other regions, identifying best practices and areas for further collaboration.