In this book, tumour growth is perceived as a deviation from the normal development of the human organism. The molecular, cellular, and tissue determinants of different tumours are discussed showing that each is a different disease, often corresponding to a particular developmental stage. The natural history of several cancers illustrates how clinical incidence can be just the visible part of the iceberg, while the first changes at the tissue level sometimes occur several years before tumour growth becomes manifest. Several mechanisms are proposed to explain the distribution of cancers during the human life span and the decline of the incidence of cancers during human senescence.