While many evaluation theorists agree that trust in the evaluation relationship is important, none have defined trust or studied the topic in depth. This grounded theory study provides an in-depth exploration of trust in evaluation relationships. In-person, semi-structured interviews were conducted with nine evaluators and nine clients of evaluation. The interviews provided rich data that inform the topic of trust in evaluation using the words of evaluators and clients. The interviews were coded and analyzed in two stages, initial coding followed by focused coding. From this analysis, ten core concepts emerged. These core concepts include personal characteristics, ability, disposition to trust, evaluation perspective, contexts, relationships, feelings, actions, outcomes, and trust as a topic. An emergent model was developed that outlines the antecedents, processes, and consequences of trust as viewed by evaluators and clients of evaluation. The importance of trust in the evaluation relationship is discussed, and further consideration of the topic is encouraged.