The idea for this volume was conceived during a discussion in the hallway at a conference in early 1990. What is the best way to detect and define pluripotent hematopoietic stem cells? was the question posed by Dr. Fritz Melchers. After discussing the pros and cons of the available assays for quite some time, it became apparent that this topic required a wider expertise and merited a larger forum. Thus, we decided to extend the discussion and to compile the results in this volume. Much to our delight. many of the pioneers of recent experimental and theoretical developments in stem cell research agreed to contribute their expertise to answer the question. These authors review both past findings and present insights, thus providing an overview of the evolution that has been and is occurring in the field of stem cell research. In the light of recent trailblazing developments in both experimental models and in clinical application it is indeed time to reevaluate our knowledge about stem cells. Trans plantation of hematopoietic stem cells has become more and more prevalent as a curative therapy in a variety of acquired and genetic diseases, including cancer, radiation accident, as an agent for gene therapy, and perhaps even as treatment for autoimmune diseases. Stem cells are now derived not only from bone marrow but also from peripheral blood, cord blood, and fetal liver, greatly increasing their availability for human transplantation and in some cases (fetal tissues) obliterating the need to match donors and hosts.