Multiscale modelling approaches can provide valuable insight to researchers across a broad range of fields, but the interdisciplinary applicability of these approaches means both new learners and experienced researchers may not have a clear understanding of the theoretical and computational fundamentals underpinning these methods. Theoretical Foundations of Multiscale Modelling aims to address this knowledge gap, providing a clear insight to the thermodynamical and statistical mechanical foundations of methods commonly employed, and highlighting the potential and limitations of each approach. Starting with an overview of the basics of statistical mechanics in Part 1, chapters illustrate the general theory of molecular modelling, the concepts and methods for the study of complex molecular systems, progressing to cover key strategies employed to perform computational calculations and simulations in Part 2. Part 3 goes on to review how the most recent, cutting-edge tools of the trade are used in practice through a selection of case studies highlighting multi-scale modelling, multiple-resolution simulation methods, and machine-learning applications. Finally, Part 4 explores the potential future of the field. Drawing on the experience of its expert authors, Theoretical Foundations of Multiscale Modelling is an insightful guide for all those learning, applying or interested in exploring multiscale modelling methods for their own work.