Lessons for critical infrastructure protection learned from the Y2K crisis--the crisis that didn't occur. A RAND study that examines the year 2000 (Y2K) crisis to assist in evaluating whether the massive and costly remediation effort was justified in view of the fact that no catastrophic system failures occurred. The report presents lessons from the Y2K experience for critical infrastructure protection (CIP) and discusses what these lessons imply for federal CIP research priorities. Spending by the U.S. government and industry on activities to prevent the predicted year 2000 (Y2K) crisis amounted to approximately 100 billion dollars, and other global spending may have been even greater. Debate continues over whether this massive effort precluded catastrophic system failures or the fears were overstated to begin with. This report presents the findings of a RAND study that attempted to shed light on this debate by addressing the following questions: What kind of event was the Y2K "crisis"? Was the massive and costly remediation effort justified? What lessons does the Y2K experience offer for critical infrastructure protection (CIP)?What do these lessons imply for federal CIP research priorities? The study included a literature review, interviews with government and industry computer experts, and a workshop involving participants in Y2K remediation efforts from industry and government. The report summarizes the workshop activities and synthesizes the key conclusions from all the project activities. It is concluded that new R&D approaches are required to deal with complex and adaptive settings. Vulnerabilities resulting from system complexity are expanding at a much faster pace than our means of understanding them. At the same time, exploitation of infrastructure vulnerabilities for criminal, terrorist, or foreign adversary purposes is a threat that potentially has no boundaries. To make CIP more manageable, research is needed that provides real data and models for understanding highly complex and uncertainty-laden environments. Such research should be a high federal priority and should be pursued aggressively.