Clinical monitoring systems that have been developed in response to the need to acquire patient physiological data have only recently been accepted as standard equipment in the operating room, and burgeoned into the variety of sophisticated patient-monitoring equipment available today. This article traces the history of the recognition of the importance of physiological data in diagnosing and treating disease, the inclusion of the four vital signs on patient charts, and the first instrumental recording of these signs, all of which occurred before 1900. The earliest operating room monitoring system, which was not developed until the mid 1940s, is described.