Adequate conceptual treatment of infant behavior requires not only suitable concepts but also factual knowledge of the behavior characterizing the infancy period. As a step in this direction, an attempt was made to describe certain clinical observations of infants in terms of one concept, that of changes in the level of tension. The criteria of a heightened state of tension developed by Lewin and his associates were found applicable to the behavior of infants under stress. Changes were observed in the following areas of functioning: posture, motility, amount of activity, readiness to startle, respiration, circulation, purposive use of objects, social responsiveness and attention span. Those aspects of functioning most affected appeared to depend on the characteristics of the infant, the nature of the stress and the severity of the tension state. Extraordinary variability in the kind of situation which aroused tension in different infants was noted. Case material was described to illustrate these points.