The concept of infant state refers to patterns of behavior that comprise the sleep-wake cycle. The infant's ability to organize state behavior rhythmically is indicative of central nervous system maturation and predictive of later development. This article describes the development of a noninvasive, computerized infant monitoring system that can be used to categorize the infant's sleep-wake behavior into states. Development of the system occurred in two phases: In the first phase of the study, reliability and validity of the infant monitoring system for recording continuous behavioral and physiological data were assessed. These results were then used in the second phase to develop a rule-based computer program to interpret the signal data stored on a micro-diskette and to characterize the infant's state throughout the period of monitoring. The capability of computerized data collection and analysis of infant state behavior has expanded the application of this infant monitoring system. Its use as an investigative tool in clinical research is discussed.