Since their widespread introduction more than half a century ago, intensive care units (ICUs) have become an integral part of the health care system. Although most ICUs are found in high-income countries, they are increasingly a feature of health care systems in low- and middle-income countries. The World Federation of Societies of Intensive and Critical Care Medicine convened a task force whose objective was to answer the question "What is an ICU?" in an internationally meaningful manner and to develop a system for stratifying ICUs on the basis of the intensity of the care they provide. We undertook a scoping review of the peer-reviewed and gray literature to assemble existing models for ICU stratification. Based on these and on discussions among task force members by teleconference and 2 face-to-face meetings, we present a proposed definition and classification of ICUs. An ICU is an organized system for the provision of care to critically ill patients that provides intensive and specialized medical and nursing care, an enhanced capacity for monitoring, and multiple modalities of physiologic organ support to sustain life during a period of life-threatening organ system insufficiency. Although an ICU is based in a defined geographic area of a hospital, its activities often extend beyond the walls of the physical space to include the emergency department, hospital ward, and follow-up clinic. A level 1 ICU is capable of providing oxygen, noninvasive monitoring, and more intensive nursing care than on a ward, whereas a level 2 ICU can provide invasive monitoring and basic life support for a short period. A level 3 ICU provides a full spectrum of monitoring and life support technologies, serves as a regional resource for the care of critically ill patients, and may play an active role in developing the specialty of intensive care through research and education. A formal definition and descriptive framework for ICUs can inform health care decision-makers in planning and measuring capacity and provide clinicians and patients with a benchmark to evaluate the level of resources available for clinical care.