There has always been the need to repair, replenish, or replace tissues and organs as the human body suffers from diseases, injuries, or simply undergoes aging. Clinically, such needs have been met with medicinal treatments, implants, and organ transplants, with various degrees of success. However, medicinal treatments often have their limits; for example, it is known that for some injuries such as thrashed muscle, if the damage is beyond a certain threshold, the tissue or organ simply will not heal. Implants have been saving lives and improving the quality of life for many patients; devices such as pace makers, heart valves, stents, and hip-joint replacements are standard treatments at present and will probably remain so for the foreseeable future. Organ transplants were a major medical advancement, the results of which range from completely restored health to at least a prolonged life. However, the availability of organs is a major issue, as are immuno logical reactions caused by complications. In the quest for solutions to such problems, a new area-regenerative medicine-has emerged as a result of rapid advancement in molecular and cell biology, physiology, chemistry, material sciences, and other relevant disciplines, and one of its main disciplines is tissue engineering. The goal of tissue engineering is “the development of functional tissues and organs in vitro for implantation in vivo or for direct remodeling and regeneration of tissue in vivo to repair, replace, preserve or enhance tissue or organ function lost due to disease, injury, or aging” (NIH 2006). Tremendous progress has been made in the past couple of decades in tissue engineering; we can now engineer bone, cartilage, blood vessels, skin, and cornea (Figure 6.1), and we can look forward to tissue-engineered liver, heart, pancreas, and other organs that will repair or replace the failed ones in patients. Another exciting development is in stem cell research. Stem cells are cells that can perpetually renew themselves but can also differentiate into multiple cell types. For example, it has been shown that mesenchymal stem cells found in our bone marrow can differentiate into osteoblasts (bone-forming cells), chondrocytes (cartilage-forming cells), myocytes (heart muscle cells), adipocytes (fat cells), and even neuronal cells. A major breakthrough took place in 2007 when Dr. Yamanaka and co-workers discovered four genes that can reprogram somatic cells such as skin cells to resemble embryonic stem cells, which opens the door to tissue engineering of organs that are genetically the patients’ own.