This book describes statistical models and methods for the analysis of longitudinal data, with a strong emphasis on applications in the biological and health sciences. The technical level of the book is roughly that of a first year postgraduate course in statistics. However, we have tried to write in such a way that readers with a lower level of technical knowledge, but experience of dealing with longitudinal data from an applied point of view, will be able to appreciate and evaluate the main ideas. Also, we hope that readers with interests across a wide spectrum of application areas will find the ideas relevant and interesting.