In this article, the authors developed several hypotheses regarding both the main and interactive effects of 2 types of team inputs on backing up behaviors in teams: (a) team composition characteristics in terms of the personality of the members of the team and (b) team task characteristics in terms of the extent to which the nature of the task is one that legitimately calls for some members of the team to back up other members of the team. Results from a study of 71 4-person teams performing a computerized tactical decision-making task suggest that the legitimacy of the need for back up has an important main effect on the extent to which team members provide assistance to and receive assistance from each other. In addition, the legitimacy of the need for back up also has important interactive effects with both the personality of the back up recipient and the personality of the back up providers on backing up behaviors in teams.