The general principles of tissue engineering involve combining living cells with a natural/synthetic support or scaffold to build a three dimensional living construct that is functionally, structurally and mechanically equal to or better than the tissue that is to be replaced. The development of such a construct requires a careful selection of four key materials: 1) scaffold, 2) growth factors, 3) extracellular matrix, and 4) cells. Much progress has been made in the field of tissue engineering, but further work toward organ and tissue replacement is necessary. The optimal cell source, scaffold design, and in vitro bioreactors, the use and development of microfabrication technology to create vascularized tissues and organs are still being investigated. The search for and use of an appropriate multipotent or pluripotent stem cell in tissue engineering is an emerging concept.