The reliability of volunteers is a major concern for many nonprofit organizations. To address this problem in more detail, we develop a theoretical model of volunteer reliability based on psychological contract theory. By taking this perspective as a starting point, we explore how individual volunteer characteristics, organizational factors, and sociological developments shape the exchange of inducements and contributions between volunteers and nonprofit organizations. We discuss how these factors can create tensions in the psychological contract and determine the extent to which volunteers behave reliably. As such, we develop a theoretical framework for addressing the reliability problem in volunteer management.