Embryonic stem cells can differentiate into cells of all three germ layers whereas adult stem cells have been traditionally considered to display a limited differentiation potential. Recently, however, a type of adult stem cells namely the mesenchymal stem cells (MSCs), have been emerged as a major breakthrough in regenerative biology. MSCs can be isolated from a variety of tissues including the bone marrow, adipose tissue and umbilical cord among others, and display the capacity to differentiate into different cell types of ectodermal, endodermal and mesodermal origin. MSCs have been therefore used in preclinical trials for tissue engineering of bone, cartilage, muscle, marrow stroma, tendon, fat, and other connective tissues. MSCs also secrete a large spectrum of bioactive molecules with immunosuppressive and immunoregulatory properties providing a regenerative microenvironment with nursing and nourishing potential. Overall, MSCs appear to be valuable tools for tissue repair and regeneration. A number of human clinical trials are now under way using allogeneic MSCs for bone and cartilage repair and treatment of myocardial infarction, stroke, spinal cord injury, graft-versus-host disease and autoimmune diseases. Tissue engineering and MSC-based scaffold technology represents also an emerging, promising field of research in regenerative medicine.