Although numerous articles advise on the training and performance enhancement of teams, the literature has neglected to account for the fact that even the most, capable teams fall to perform at optimal levels. Such decreases in level of performance are what we refer to as team performance decay. Although the common practice has been to respond to decreases in performance after they have occurred, we argue this approach to be insufficient, especially given the number of teams that currently operate in critical, high risk environments, where performance slips may be fatal for either the team members or others (e.g., medical teams, fire crews, military units). It would be advantageous to battle team performance decay proactively, thus, circumventing the consequences of performance decay; however, to do this, we must first understand the sources of performance decrement. Therefore, the present chapter addresses the concept of team performance decay and the factors at the environmental, task, and team level associated with its occurrence. However, before discussing performance decay and its sources, we must first consider team performance itself.