In my dissertation consisting of three essays related to observer reactions to workplace mistreatment, I use three different approaches to gain a deeper understanding of when and how observers will intervene in workplace mistreatment incidents. Because many targets do not respond actively to mistreatment and prefer to respond passively, observers can play a vital role in stopping and preventing future mistreatment on behalf of targets. In the first paper, I used a qualitative interview approach and examined 71 incidents of interpersonal workplace mistreatment experiences. I found that observers intervened either through social interpersonal relationships or through power-based hierarchical procedures. Some common factors that influenced observer reactions included concern for actors’ welfare, perception of intervention effectiveness, uncertainty about mistreatment allegations, perceived harm level, power, job role and responsibility, and reporting accessibility. In the second paper, I developed a theoretical model to explain why a majority of observers often do not intervene and report mistreatment in organizations. Specifically, the model explains how target reactions, perceptions of whether intervention and mistreatment are legitimate, whether targets or perpetrators permit intervention, and organizational disciplinary systems can influence whether third-party employee observers will intervene or not.In the third paper, we conduct an experimental study using economic behavioral games to examine how the presence of formal justice systems (weak, strong, perfect, or no system) influence instigator wrongdoing, target reporting, and observer helping behaviors. We find that the absence of a formal system is better than a weak formal system, and that there is not a big difference between a weak and strong formal system. Only a “perfect” formal justice system is the best at preventing instigator wrongdoing and encouraging observer helping behaviors.