In 1890, just a few years after the discovery of the chromosomes, David Paul Hansemann, a pathologist-in-training with the famous Rudolph Virchow in Berlin, produced a theory of the pathogenesis of cancer involving the key current concept: that the first change which occurs in cancer is an alteration of the hereditary material of a normal cell at the site where the cancerous process begins. In the process of linking cancer to chromosomal material, Hansemann coined the terms 'anaplasia' and 'dedifferentiation'. These terms have remained the basis of descriptive terms concerning the microscopical appearances of tumours ever since. Nevertheless, despite the popularity of his terminology, Hansemann's ideas were attacked vigorously by almost all proponents of rival theories of the nature of cancer. Partly due to these disputes during his life-time, and partly due to other factors, interest in von Hansemann's ideas diminished during the twentieth century and his works are rarely mentioned today. This book presents translations of all the relevant German texts, and analyses the background and context of Hansemann's theories as well as the reasons why he was almost completely forgotten. It shows that some of Hansemanns ideas may still be relevant to cancer research today, and that he deserves to be remembered in relation to cancer as Vordenker unter den führenden Denkern seiner Zeit - The foremost of the leading thinkers of his time.