The study of cultured human tumor cells is a most obvious approach in experimental human cancer research. For many techniques in virology, immunology, biochemistry, and biophysics, for example, large amounts of cells may be required and such quantities are usually provided only when the cultures develop into established cell lines; when this happens, thorough characterization also becomes possible. The development of cell lines, therefore, is of prime importance. Recent major advances in research with animal cell systems see m to be a prologue for present and future efforts directed toward work with human tumor cells in culture. Conceivably, the most significant results in cancer research may develop from work with such cells, and so the time seemed right to define the present state of our knowledge. This is the first book dedicated exclusively to the subject: human tumor cells in vitro. Although so me of the fundamental aspects in the cultivation of human tumor ceIls, and the extent to which they represent human cancer in vivo are still unclear, I asked a number of the leading investigators in this area of research to collect and evaluate previous and present contributions, and to offer their thoughts on the questions to which answers are not yet available. Many of the chapters are concerned with techniques of cultivation. Cultures from some types of tumors have grown weIl; in many cases they have given rise to established cell lines.