Stem cells are self-renewing, unspecialised cells that can give rise to multiple cell types of all tissues of the body. They can be derived from the embryo, foetus and adult. The ability of stem cells to divide but also to differentiate to specialised cell types like nerve and muscle, have made them candidates on which to base therapies for diseases and disorders for which no, or only partially effective, therapies are available. Replacement of defective or absent cells in defective tissues and organs could represent a cure. Here, we introduce the background to stem cell research and review the present state-of-the-art in stem cell biology, directed differentiation and tissue repair. In particular, we distinguish embryonic versus adult sources of stem cells and data derived from animal versus human experiments in order to place current research and perspectives for clinical application in their correct context.