Abstract Mesenchymal stem cells (MSC), found in the bone-marrow, the blood circulation, adipose tissue and fetal lungs, have recently been subject to great interest from researchers and clinicians worldwide. These cells provide the body with progenitors for mesodermal derived tissues such as bone, cartilage, muscles, fat and haematopoietic stroma. Whether MSCs also give rise to cells from other embryological derms, the ekto- and endoderm, is under investigation. Suggested explanations why MSCs seem to differentiate into cells of non-mesodermal tissues include transdifferentiation, de-and re-differentiation and cell-fusion. The existence of a pluripotent cell like the ones found in early embryological life is another possibility. How the MSCs function in vivo is subject to discussion. The cells have experimentally been shown to incorporate into certain tissues as functional cells. In yet other sites MSCs provide damaged tissues with humoral factors stimulating tissue repair. MSCs used in therapeutic medicine can be autologous cells (from the patient himself) or allogenous (from another individual). They can be mobilized to the circulation from the bone-marrow by colony stimulating factors or be extracted directly from tissues. MSCs harvested from the body can then be subject to manipulation ex vivo or simply undergo expansion. The cells will then, after intravenous infusion, find their way to the harmed tissues probably by different homing mechanisms. MSCs can also be transplanted directly into the tissue of interest. This paper will discuss the above and in addition include several studies on the potential use of MSCs in clinical practise.