This thesis investigates limits of biological knowledge at the example of stem cell research; on the one hand by exploring limitations often discussed in terms of limits of reductionism, i.e. those arising from tensions between methods of experimental life science research and peculiarities of the living world such as biological organisation, on the other hand by investigating possible epistemic consequences of the focus on biomedical or biotechnological application. Such a philosophical analysis is relevant for both science and society. The high expectations concerning especially stem cell research shared by wider society, including many scientists and ethicists, will be shown to rely partly on biologically rather implausible concepts of stem cells and of the possible scope of knowledge and control achievable over them. Thus this project aims in addition to contributing to key debates in philosophy of biology at ‘social relevance’ by throwing a light into the thicket of hope and hype surrounding stem cell research and by providing resources for ethicists interested in reflecting assumptions about scope, functioning and objects of stem cell research underlying their respective debates and positions.