The world of experimental leukaemia research is a miniature of cancer research. Cancer research started from experimental leukaemia/sarcoma research of Ellermann and Bang and Peyton Rous in early 1900s. Experimental leukaemia includes most of important elements of cancer research; both viral and chemical carcinogenesis; most of important oncogenes and suppresser genes and the related cell biology. The author has added only a few sections such as skin carcinogenesis done by Yamagiwa and Berenblum and DNA viruses from outside of experimental leukaemia. By doing this, this book covers most of important topics of modern oncology, except researches done on human cancers. In this book, the author has arranged the chapters to contrast the pioneer works by Peyton Rous, Jacob Furth, and Ludwik Gross, etc, done more than half a century ago and the recent works by molecular biologists. Although the former pathologists provided various experimental materials, faced to severe criticism by contemporary researchers and they had tough hearts to cope with them, molecular biologists solved questions on these materials step by step by disclosing the invisible molecular events as a visible charts, molecular diagrams, or 3-D models just as magicians do. Experimental leukaemia has always afforded the key materials for solution of cancer. In experimental leukaemia, mutations and genetic recombination play an important role. They include ras gene mutations, insertional mutagenesis and gene transduction by retroviruses, recombination among retroviruses, and chromosome translocations. Most of them occur in highly specific genomic sites. However, the reason for the specificity in target sites remained obscure. The author was especially interested in the common target genomic sites for chemical carcinogens and retroviruses. Therefore the common pathway for retroviruses and chemicals became one of the main topics of this book.