It's one of the oldest jokes on the Internet, endlessly forwarded from e-mailbox to e-mailbox. A software mogul-usually Bill Gates, but sometimes another-makes a speech. "If the automobile industry had developed like the software industry," the mogul proclaims, "we would all be driving $25 cars that get 1,000 miles to the gallon." To which an automobile executive retorts, "Yeah, and if cars were like software, they would crash twice a day for no reason, and when you called for service, they'd tell you to reinstall the engine."

The joke encapsulates one of the great puzzles of contemporary technology. In an amazingly short time, software has become critical to almost every aspect of modern life. From bank vaults to city stoplights, from telephone networks to DVD players, from automobile air bags to air traffic control systems, the world around us is regulated by code. Yet much software simply doesn't work reliably: ask anyone who has watched a computer screen flush blue, wiping out hours of effort. All too often, software engineers say, code is bloated, ugly, inefficient and poorly designed; even when programs do function correctly, users find them too hard to understand. Groaning beneath the weight of bricklike manuals, bookstore shelves across the nation testify to the perduring dysfunctionality of software.