In 2000, Tony Payne and I organized a session at the annual congress of the European Geophysical Society on the mass balance of the cryosphere. It was clear from the impressive scientific breakthroughs presented at this meeting and also in the recent literature that major progress has been achieved in this subject over the last decade. This is a result of advances in both observational technology through new satellite and airborne hardware, and our modelling capability, and through improvements in computational power and physical understanding. As a consequence, it was timely and fitting to embark on producing a comprehensive review of what we know about the theory behind measuring and modelling mass balance and the actual results from the latest observations and model simulations. In this respect, the book is unique, in that it combines both the theory and the results in a single text. Twenty-three expert authors have contributed to seventeen chapters covering sea ice, glaciers and ice sheets in five thematic sections. Although this is an edited volume, each chapter is extensively cross-referenced and forms part of a fully integrated text. In addition, the chapters were externally peer-reviewed to ensure the highest scientific standards. Part I of this book is designed to offer a comprehensive, yet compact, reference text on the theory and practice of measuring mass balance.