Two major evolutions have been reported to occur in the nonprofit sector during the past decades. Both the nature of nonprofit organizations (NPOs) and of volunteering style are changing. While this creates challenges for NPO governance and management, little is known about the process or the outcomes of these two developments. We propose a two-dimensional conceptual model to explain how the aforementioned evolutions influence the attitudes and behavior of volunteers, based on psychological contract theory. More specifically, we posit that both evolutions create tensions in volunteers’ psychological contracts that could lead to contract breach. We formulate twelve propositions on the nature of this psychological contract breach and the resulting attitudes and behavior of volunteers. Finally, we offer some possible solutions that NPO boards and managers can apply to cope with these challenges.