There is little agreement on sleep-wake state categories for describing infants, nor is there agreement on how states are to be conceptualized. We propose General Systems Theory as a perspective for viewing behavioral states and for describing their function as a behavioral system within the infant's larger social system. In the social context, states provide communicative cues, mediate perception of caregiver stimuli, and modulate the infant's responses to those stimuli. Thus, states reflect the infant's CNS status and they are related in a complex, dynamic way to the developmental course of that status. A taxonomy of infants' behavioral states is described, composed of ten Primary States: Alert, Nonalert Waking, Fuss, Cry, Drowse, Daze, Sleep-Wake Transition, Active Sleep, Active-Quiet Transition, and Quiet Sleep. Using combinations of these states, a set of six Derived States is also defined. Results of naturalistic studies of infants' states are described to demonstrate for each of these states: reliability of measurement, evidence for concurrent and predictive validity, and unique intraperson correlation patterns. The findings support the usefulness of this state taxonomy for describing infants and for investigating the functions of state. Categorical and conceptual differences with the states as viewed by Wolff and Prechtl are discussed.