Induced pluripotent stem (iPS) cells can be generated by forced expression of four pluripotency factors in somatic cells. This has received much attention in recent years since it may offer us a promising donor cell source for cell transplantation therapy. There has been great progress in iPS cell research in the past few years. However, several issues need to be further addressed in the near future before the clinical application of iPS cells, like the immunogenicity of iPS cells, the variability of differentiation potential and most importantly tumor formation of the iPS derivative cells. Here, we review recent progress in research into the pluripotency of iPS cells.