Stem cell biology is now one of the most exciting and rapidly advancing areas of scientific endeavor. Promises of cures of a wide variety of diseases by specific replacement of damaged or malfunctional tissues by use of totipotent or multipotent stem cells is on the horizon in clinical practice. Stem cells derived from the embryo and from adult tissues have been shown to have extensive potentials for self-renewal and differentiation. In addition, the plasticities of phenotype exhibited in vivo by some of these cell populations challenge the doctrine of irreversibility of cell commitment after particular developmental stages. This brief review considers certain aspects of these recent findings of the many unexpected potentials of stem cells to differentiate into alternative processes, and their potential value for use in tissue reconstruction procedures are prominent areas that require further study. Rigorous investigation of these topics will lead to realistic approaches in the future for stem cell therapy in a variety of human diseases and other clinical problems.