Scientific and philosophical inquiry often assumes that systems can be studied in isolation, yet real-world systems are never truly closed. They constantly interact with their environment, exchanging energy, information, and matter. This openness challenges fundamental assumptions about determinism, causality, and emergence, raising profound questions about how we model, explain, and understand the world. This book brings together leading philosophers and scientists to explore the implications of open systems across physics, metaphysics, and methodology. How do interactions shape the behaviour of physical systems? What are the consequences of idealizing real systems as closed? How does openness affect our understanding of fundamental theories, such as quantum mechanics and statistical physics? And what does it mean for the broader philosophical concepts of reduction, explanation, and emergence? At the heart of this volume lies the recognition that open systems require a shift in perspective-one that acknowledges the limits of traditional approaches and embraces new ways of thinking about complex, dynamic systems. Through interdisciplinary contributions, the book offers fresh insights into topics such as non-unitary evolution in quantum mechanics, the role of decoherence in the quantum-to-classical transition, and the challenges of modeling open systems in scientific practice. Written for scholars and students in philosophy and physics, as well as anyone interested in the foundations of science, Open Systems: Physics, Metaphysics, and Methodology invites readers to rethink fundamental concepts in light of the inherent openness of the systems that shape our world.