Recent findings have placed stem cell research at the forefront of biomedical sciences. Basic research on embryonic stem cells (ESCs) has contributed to our knowledge about the developmental potential and plasticity of stem cells. Furthermore, it has raised hope to use these cells as potential source for regenerative medicine and tissue replacement after injury or disease. Unfortunately, ESCs can also form tumors and they are ethically controversial because they originate from human embryos. This review summarizes findings and therapeutic applications of ESCs and their alternatives: adult stem cells and iPS cells.