When this volume was first published in 1998, drug resistance in cancer, whereby a proportion of cancer cells evades chemotherapy, poses a profound and continuing challenge for the effective treatment of cancer. The principles underlying the biological mechanisms behind this phenomenon are clearly understood and explained in this volume. However, a deeper understanding of drug resistance requires a quantitative appreciation of the dynamic forces that shape tumour growth, including spontaneous mutation and selection processes. The authors seek to explain and to simplify these complex mechanisms, and to place them in a clinical context. Clearly explained mathematical models are used to illustrate the biological principles and provide an insight into tumour development and the effectiveness and limitations of drug treatment. It is suitable for those with a non-mathematical background and aims to enhance the effectiveness of cancer therapy.