Background and Objectives: Attentional Control Theory (ACT) predicts that trait anxiety and situation stress combine to reduce performance efficiency on tasks requiring rapid shifts in attention. Recent evidence has also suggested that working memory capacity (WMC) might moderate this relationship. We controlled for methodological difficulties in the existing literature to investigate the relationships between trait anxiety, situational stress, and WMC on attentional shifting. Design and Method: Seventy undergraduate students participated in the study. Trait anxiety was operationalized using questionnaire scores, situational stress was manipulated through a pressured counting task, and WMC was based on performance on the Automated Operation Span Task (AOSPAN). The shifting task involved a modified version of the Sternberg paradigm as the primary task and an oddball tone-discrimination task as the secondary task. Dependent variables were performance effectiveness (accuracy) and processing efficiency (accuracy divided by response time) on the secondary task. Results: There was no effect of anxiety, stress, or WMC in predicting performance effectiveness; however, a significant three-way interaction on processing efficiency was observed. At higher WMC, anxiety and situational stress were not associated with processing efficiency. Conversely, at lower WMC, higher trait anxiety was associated with poorer efficiency but only for those who reported higher situational stress; for those who reported lower situational stress higher trait anxiety predicted facilitated efficiency. Conclusions: Results are interpreted with respect to ACT and directions for future research are discussed.