As the author started working on this book it dawned on him that the vast experimental and clinical material on the metabolism and biological properties of tumours accumulated by modern science can be properly appraised and analysed only by going beyond the scope of his narrow speciality of a biochemist and by attempting to comprehend a number of phenomena and touching upon problems of morphology, genetics, pathophysiology, immunology and endocrinology. The author tried to be precise and to separate the substantiated, indisputable data and generalizations from the hypotheses still requiring verification, however attractive they may at first sight be.