Abstract: Media reports have testified to the frequency and profound consequences of workplace aggression, focusing attention on this behaviour. Workplace aggression involves behaviour with an intention to harm others in a work-related context. In this dissertation, I used the General Aggression Model to guide three studies examining employee responses to experiencing mistreatment in the workplace. Study 1 examined frequency and intensity indices of workplace mistreatment in a mediation model of experienced incivility predicting enacted aggression, and in turn, enacted aggression three months later. This study found that these indices showed different results. Frequency showed full mediation, suggesting that if frequency of aggression does not escalate right away (at Time 1), it will not escalate in the longer term. Intensity, on the other hand, showed a partial mediation, suggesting that intensity of aggression can increase in intensity right away or in the longer term three months later. Further, moderation results suggested that stronger climates of mistreatment moderated the aggression frequency and intensity relationships, buffering the likelihood of aggressive retaliation. Anger rumination moderation results suggested that employees retaliate aggressively following experiencing incivility from a coworker soon after the incivility, but not in the longer term, three months later. A longitudinal employee survey methodology in Study 2 found that employees used negative (aggressive) and positive (organizational citizenship behaviours) behaviours to deal with social identity threats from those outside the organization (e.g., customers) targeted to a group of theirs, namely their organization. This study also found that a moderating mechanism, organizational identification buffered aggressive responses following social identity threat. Further, employees also used aggression as a response to experiencing personal identity threats directed towards them as an individual. Study 3 employed a longitudinal design with three time points to examine direct retaliatory aggression with customers and displaced aggression from organizational insiders towards customers. Results suggested that aggression that occurs inside the organization does not stay within the organization, as indicated by significant displaced aggression results. The three studies in this dissertation suggest that employees respond to mistreatment several ways, and moderating mechanisms suggest that organizations can take steps in reduce workplace aggression.