Several fundamentally important questions form the basis for this book. What are the relationships between tumour formation and tumour pH? What are the effects of tumour pH and hypoxia on carcinogenesis or tumorigenesis? What are the therapeutic consequences of tumour pH? It is hypothesised that low extracellular pH is not only an important consequence of tumour growth but may also promote further tumorigenic transformation. Furthermore, in vitro studies suggest that low pH strongly affects the efficacy of chemo- and radiotherapy. Better understanding of the influence of pH on tumour growth, coupled with manipulation of the pH of the tumour microenvironment, may lead to the development of more effective therapies.