Instant messaging is an extremely useful tool, since it combines the intelligence that comes from presence information - knowing when and where someone can be contacted - with the ability to share text or files, hold informal meetings, and do other useful stuff quickly and easily. But how do you justify the rollout of an IM infrastructure on the basis of \u201cgreater productivity\u201d or \u201cdoing things more easily or more quickly\u201d?IT decision makers typically like \u201chard\u201d ROI numbers, such as reductions in the amounts of the bills they pay, like telephone bills, travel expenses, and so forth.We\u2019ve done some calculations on the cost savings provided by IM, and have reviewed internal cost savings provided by others, and have come to a conclusion with which some will probably disagree: justifying IM in the enterprise will be most successful when it changes people\u2019s behavior, not when it simply reduces the use of the telephone or e-mail.For example, let\u2019s look at the cost of deploying an IM infrastructure for 500 users. While the numbers can vary significantly depending on the system chosen, our estimate is that the full deployment cost of an enterprise-grade IM system for this many users will initially cost about $175 per user, and that ongoing maintenance of the system will cost a little over $5 per year.If we assume that the IM system will save three long-distance telephone calls per week and will result in a 5% reduction in e-mail storage requirements, it will take more than five years to provide a return on the initial and ongoing investment of the IM infrastructure using hard costs alone. However, if the IM system, in conjunction with Web conferencing, could result in employees traveling less and instead meeting electronically, the payback period could be shortened dramatically.The bottom line is that for IM to be justified on the basis of hard costs alone, it must provide changes in business processes that are significant enough to justify its deployment. Incremental changes or minor cost reductions just may not be enough for some IT decision makers to believe that IM is worth the time or expense, especially in the context of competing demands for limited IT dollars.