Chapter 4 gives a basic review of the fair process effect. Research conducted in laboratory settings is able to study the fair process effect with causal control. This has revealed insights into basic reasons why the fair process effect is important for people. Studies done in work organizations typically have more difficulty showing the causal quality of the effect, but are important in showing the relevance of positive associations between employees’ perceptions of procedural fairness and other reactions, such as trust in management and organizational citizenship behavior. Similarly, research in legal settings indicates that citizens are more willing to trust the legal system and voluntarily accept decisions by important legal authorities, such as police officers and judges. Related to this, in various societal contexts, high levels of perceived procedural fairness are associated with positive reactions and constructive behaviors, such as voting in democratic elections and efforts to improve one’s community. In contrast, low levels of perceived procedural fairness are linked with negative reactions and destructive behaviors, such as radicalization into violent extremism and terrorism. This chapter also warns against unwarranted simplifications about the fair process effect, thereby adopting a realistic perspective when combating fundamental distrust, polarization, and conspiracy thinking.