Examining stem cell biology from a philosophy of science perspective, this book clarifies the field's central concept, the stem cell, as well as its aims, methods, models, explanations and evidential challenges. The first chapters discuss what stem cells are, how experiments identify them, and why these two issues cannot be completely separated. The basic concepts, methods and structure of the field are set out, as well as key limitations and challenges. The second part of the book shows how rigorous explanations emerge from stem cell experiments, and compares these to other kinds of scientific explanation. Model organisms, the role of genes, and the significance of collaboration are also discussed. The last part of the book considers relations to systems biology and clinical medicine, arguing that both the mathematical models of the former, and ethical principles of the latter, are necessary for stem cell biology to deliver on its promises --