This book will equip a student of any physical or biological science with a sound understanding of thermodynamics, and will build confidence in using thermodynamics in practice. The emphasis is towards chemical thermodynamics, but the principles of the First, Second and Third Laws apply to all sciences. Importantly, the final four chapters show how thermodynamics can be applied to biological systems, discussing the biochemical standard state, bioenergetics, protein folding, and the self-assembly of smaller components to form higher-level structures. The book has not been written to support a particular curriculum; rather, it covers all the fundamental principles, so providing a comprehensive grounding, as well as a strong foundation for further study. It is therefore likely that there will be more material in this book than is required for any one particular curriculum, but we trust there is sufficient material for almost every curriculum. A key feature of the book is the style. It has been written so that ‘you can hear our voices’, and with the overarching intent of being logical, clear and comprehensible. The style will therefore be perceived as less formal than many other texts – and we trust more readable. Furthermore, we have sought to avoid phrases such as ‘it may be shown that...’, and ‘clearly, it follows that...’. If ‘it may be shown’, we show it; and we don’t use ‘clearly’ when things aren’t clear at all. Thermodynamics is notoriously difficult. This book does not make an intrinsically deep science ‘easy’. But it does make it intelligible.