There are more than 100 million instant-messaging users, the majority employing free services provided by AOL, Yahoo and MSN. But most of IM's growth lies ahead. As the technology expands into enterprise and wireless applications, the number of IM users worldwide easily could top 500 million.There are already millions of enterprise IM users. Users who need to touch base frequently with the same co-workers often prefer IM to the phone and e-mail. IM is faster and more convenient.But that's just the beginning. "Real-time enterprise" applications will use IM for machine-to-person, person-to-machine, and even machine-to-machine communications. For example, thanks to IM's "presence" feature, an application automatically can find and contact the best available person to handle a specific problem.As companies deploy IM products they will want to extend them to their mobile users. Mobile phone operators have discovered they can obtain far higher revenue per megabyte from messaging services than from generic IP services. Text messaging via mobile phones is already popular in Europe and is growing rapidly in North America. The opportunity to expand and enhance wireless messaging is huge.Until now, mobile text messaging has relied on a store-and-forward technology:\u00a0Short Message Service. Wireless IM will let operators take mobile messaging to the next level, delivering more powerful services integrating presence, location (where are my buddies right now?), sophisticated user preferences (who is allowed to reach me during work hours?) and multiple media (text with audio, images and video).But there are significant challenges. The first wireless IM products simply extend desktop IM to mobile phones. AOL, Yahoo and MSN use proprietary protocols and are not interoperable. But the allure of wireless IM is the promise of reaching anyone, anywhere. That will require interoperability between mobile phone operators, desktop IM service providers and enterprise systems.The four largest mobile phone operators in the U.S. - Verizon Wireless, Cingular Wireless, AT&T Wireless and Sprint PCS - all have introduced wireless IM services. Verizon and Sprint PCS support AOL, MSN and Yahoo. AT&T Wireless supports AOL and Yahoo; Cingular only supports AOL. Wireless IM typically costs $5 per month for a bucket of messages, or users can pay 10 cents per out-bound and 2 cents per in-bound message.In the future, leading IM services will employ presence-enabled address books. Address books automatically will show the availability of people, perhaps including their location, and will let users specify their preferences (for example, text while in a meeting and location information only while working).The demand for more powerful ways of keeping in touch continues to grow - even as users complain about loss of privacy. However, the two are not mutually exclusive. More powerful means of keeping in touch also can provide more powerful controls. With wireless IM, we can be in touch precisely where we want, when we want, with whom we want and in the mode we want.