Mesenchymal stem cells can be isolated from human bone marrow aspirates, and, thanks to their differentiation potential and good in vitro culture properties, represent an attractive cell line for the replacement of mesenchymal tissue, i. e. tissue engineering. Both in vitro and in vivo, they can differentiate into cartilage, bone, tendons and fat cells, and--in contrast to embryonic stem cells--are not underethical scrutiny. Cultured in three-dimensions on a variety of carrier substances, they have already been successfully employed for the reconstruction of mesenchymal tissues in numerous studies involving both small and large animal models. While a clinical application in humans has also been successful in pilot studies, numerous points still remain to be clarified, underscoring the need for further intensive research before wide-scale clinical application can be contemplated. Only then can it be shown whether the associated high expectations are justified.