Objectives: The concept of severity in irritable bowel syndrome (IBS) is clinically recognized and operative in diagnostic decision making and treatment planning. Yet, there is no consensus on its definition, and there are limited data on the prevalence of severity subgroups, its medical and psychosocial determinants, and its association with other health status measures. The aims of the Rome Foundation Working Team Committee were to summarize current research, to develop a consensus of understanding on this concept, and to make recommendations for its use in research and clinical care. Methods: In 2006, a multinational committee of clinical investigators with expertise in IBS and/or psychometric research methods undertook a systematic review of the literature relating to severity in IBS. Owing to limited data, the Foundation commissioned three clinical studies to better characterize the concept of severity in IBS, and summary information and recommendations for future research and clinical care were developed. Results: The main findings were: (i) severity in IBS is defined as a biopsychosocial composite of patient-reported gastrointestinal and extraintestinal symptoms, degree of disability, and illness-related perceptions and behaviors; (ii) both visceral and central nervous system physiological factors affect severity; as severity increases, the central nervous system provides a greater contribution; (iii) severity is related to and influences health-related quality of life and health behaviors and also guides diagnostic and therapeutic clinical decision making; (iv) severity can be subcategorized into clinically meaningful subgroups as mild (∼40%), moderate (∼35%), and severe (∼25%), and this provides a working model for use in future research and clinical care. Conclusions: Future work is required to understand more precisely the factors contributing to severity and to develop a valid patient-reported instrument to measure severity in IBS. © 2011 by the American College of Gastroenterology.