Little explicit attention has been given to the potential impact of client challenging behavior on staff psychological well-being. Relevant research was critically reviewed according to criteria required to establish a causal relationship. Reasonable evidence was found for an association between challenging behavior and staff stress, and some evidence for temporal precedence. However, few researchers had analyzed data enabling alternative explanations to be eliminated. Thus, empirical evidence of a causal relationship between challenging behavior and staff stress is weak. A fourth causality criterion was addressed by suggestion of a mechanism by which challenging behavior may have its effects. I propose that staff negative emotional reactions mediate the impact of challenging behaviors on staff stress and evaluate research evidence for this theory.