When Apple launched iTunes, music labels were desperately struggling with the impact of digital technology. CD sales were declining as fans grabbed music from file-sharing sites. Between 2000 and 2006, according to the Recording Industry Association of America, CD shipments tumbled 35 percent—from 942.5 million to 614.9 million. Executives at record companies should have worried that things would get worse, yet their attitude seemed disconnected from reality. About 10 years ago, I listened in disbelief as a top music executive asserted that people like the experience of going into stores “to see and touch” CDs and prefer to get their music that way. This view remained widespread even as illegal downloading of music went mainstream.

Meanwhile, Apple was enjoying a strong position in the computer industry but found that it needed to look beyond its native market in order to create growth. The company’s tiny market share suggested that sales of its somewhat pricey products had peaked. As an established brand, Apple could afford to develop a digital distribution system and attack the entrenched players. Startups like Napster could not, and they were easily thwarted by the record companies.

Interestingly, Apple also had an advantage in C.E.O. Steve Jobs. Executives of potential XBDs are often blind to cross-boundary opportunities because they tend to focus only on their own markets—even though other industries may offer huge potential. Burgelman and I call this the disrupter’s paradox. Those who are strong enough to mount an attack on another industry will rarely be aware of the opportunity to do so. Jobs, however, had a long involvement with the media industry through his investment in Pixar Animation Studios, the creator of Toy Story and other megahits. This experience most likely helped him avoid the disrupter's paradox.

Since Apple introduced the iPod and iTunes, it has sold more than 100 million of the digital music players and more than 3 billion song downloads. Profits have soared. Meanwhile, CD sales continue to fall. In the first six months of 2007, they dropped 19 percent, according to Nielsen SoundScan.

Digital technology is bound to facilitate other cross-boundary disruptions. Google’s move into advertising with its AdSense program may prove to have a revolutionary impact on advertising, not only because it diverts revenue away from traditional media, but also because it allows for increasingly well-targeted context-sensitive ads in which the commercial message is presented to readers or audiences at precisely the right time and closely matches their interests. Whoever has a good understanding of these possibilities and can move aggressively might redefine the business of advertising and, by extension, the media that carry commercial messages. It’s not surprising that a rule breaker like Google is driving this change.

In my email to Scott, I wrote that transforming the health-care industry could become "the most appropriate emerging example" of cross-boundary disruption. I should have said most important. The health-care industry ultimately helps define each of our lives, our children’s well-being, and the way we spend our golden years. It is huge, big enough to provide fertile ground for any would-be XBD. In the U.S., this market is worth an estimated $2.26 trillion, more than six times the size of Wal-Mart, which reported revenue of $349 billion last year. The industry's structure is inscrutable. Almost anyone who has had to navigate physician networks, hospital chains, and giant insurance companies ends up wishing that somebody would do something to fix health care. What’s more, the medical system’s willingness—and ability—to change is questionable. It should be an attractive target for a player from another industry with the resources and core competencies to attack it.

Wal-Mart is in an excellent position to assume the role of the disrupter. It has an incredible record of innovation and execution, yet it has grown to the point where its very success has led to problems. Wall Street analysts characterize this giant as having saturated its market with more than 4,000 stores. Same-store sales have fallen from year to year, and Wal-Mart's share price has dropped from its all-time high of $69, in December 1999, to $45 in October.

The need to do something different must have weighed on management, and its solution may prove to be a new disruptive phenomenon. Two years ago, Wal-Mart launched a pilot program of in-store clinics. These facilities provide standard medical treatments and services, ranging from vaccines for measles to cholesterol screening, all at low fixed prices. A standard checkup, for example, tops out at $65. Since 2005, Wal-Mart has opened 78 clinics in 13 states and has plans for up to 400 more in the next three years. If these succeed, the company intends to build up to 2,000 by 2014.