At 79, thomas starzl doesn't have much left to prove. In July 1967, the surgeon was the first to perform a successful human liver transplantation; since then he has been instrumental in making the liver the United States' second-most-commonly transplanted organ. "He led the field of transplantation into the modern era," says J. Richard Thistlethwaite, a transplant surgeon at the University of Chicago.

Despite his accomplishments and acclaim, though, Starzl still goes to work each day at the nation's most active organ transplantation center, a University of Pittsburgh fa­cility that happens to be named after him. He and others in the field, it turns out, are still struggling with the same basic challenge they've faced since Starzl's pioneering surgeries: preventing patients' immune systems from attacking and destroying new organs. That a field's founder is still doing battle with some of its oldest demons more than 40 years later is instructive. It reminds us that the birth of a new technology is not a discrete event, and that emerging technologies bring not only hope and new possibilities but also their own collections of emerging problems. That may be frustrating, but it's what drives many innovators and keeps them coming back to the office long after they really need to. "I'm pretending to be retired," says Starzl.