The phrase, 'hospital information system', is frequently used in discussions about the flow of information throughout a hospital with the assumption that everybody has the same concept in mind. Closer examination shows that this is not necessarily the case. The author draws on his experience as the Chief Information Officer at University Hospital at Stony Brook to define a hospital information system in terms of the implementation at Stony Brook. The University Hospital Information System at University Hospital (UHIS), has received international acclaim and was recently selected by the IBM Quarterly of Australia as the world leader in hospital information systems. This paper answers four questions: What is a hospital information system? How does a hospital information system work? How do you implement a hospital information system? After the system is operational, where do you go, e.g., critical care data management, physician's office management? University Hospital at Stony Brook is located on eastern Long Island and is the tertiary care referral hospital for approximately 1.4 million people. Nothing in the hospital happens without computers. Doctors, nurses, administrators and staff at all levels rely on the system daily. The system operates 24 hours per day, seven days per week. Access to the system is through 300 terminals and 128 printers throughout the hospital. In addition to the UHIS terminals, the critical care management system which is called Patient Data Management System, (PDMS), is available at over 90 ICU beds and in the operating rooms.