When two of us (Langer and Vacanti) last wrote in this magazine 10 years ago about prospects for tissue engineering, the very idea that living flesh could be “constructed” by following engineering principles and combining nonliving materials with cells sounded fantastical to many. Yet the need for such transplantable human tissues to replace, restore or enhance organ function was, and remains, urgent. Today nearly 50 million people in the U.S. are alive because of various forms of artificial organ therapy, and one in every five people older than 65 in developed nations is very likely to benefit from organ replacement technology during the remainder of their lives.