In this dissertation I develop and test a conceptual model of how positive affect shapes project team development and effectiveness. Integrating and extending the literatures on positive affect and project team development, I propose that team trait positive affect—the relatively stable collective tendency for the members of a team to experience shared positive moods over time—aids in the development of three resources that research suggests are critical for project team effectiveness: team task routines, friendship network density, and team efficacy. Specifically, I suggest that team trait positive affect—a stable team “disposition” built from team members’ relatively homogeneous trait positive affect—shapes team developmental trajectories—the paths that a team takes in building resources over the course of its lifespan. Extending the literature on team development and effectiveness, I propose that team developmental trajectories with respect to task routines, friendship density, and team efficacy are key drivers of team performance. As such, I argue that positive affect indirectly shapes project team effectiveness by influencing patterns of team resource development over time. I find empirical support for key components of my conceptual model in a longitudinal survey-based study of 33 military teams preparing for and participating in a military competition. Team trait positive affect, my findings suggest, has important and significant effects on how teams develop over time, and furthermore, on team effectiveness. Indeed, while team trait positive affect relates to team effectiveness indirectly through its relationship with team developmental trajectories, the impact of team trait positive affect on team effectiveness is most clearly strong and direct. In total, my theory and my findings suggest that team trait positive affect is a critical variable for understanding project team development and effectiveness.