To describe three levels of hospital-based critical care centers to optimally match services and personnel with community needs, and to recommend essential intensive care unit services and personnel for each critical care level. A multidisciplinary writing panel of professionals with expertise in the clinical practice of critical care medicine working under the direction of the American College of Critical Care Medicine (ACCM). Relevant medical literature was accessed through a systematic Medline search and synthesized by the ACCM writing panel, a multidisciplinary group of critical care experts. Consensus for the final written document was reached through collaboration in meetings and through electronic communication modalities. Literature cited included previously written guidelines from the ACCM, published expert opinion and statements from official organizations, published review articles, and nonrandomized, historical cohort investigations. With this background, the ACCM writing panel described a three-tiered system of intensive care units determined by service-based criteria. Guidelines for optimal intensive care unit services and personnel for hospitals with varying resources will facilitate both local and regional delivery of consistent and excellent care to critically ill patients.