This chapter describes the development of fairness theory (FT) and research based on it. FT deals with the perception of (un)fairness based on counterfactual thinking. FT grew out of referent cognitions theory (RCT), which focused on circumstances creating resentment. The chapter explains the transition from RCT to FT and their differences. Folger and Cropanzano (1998) introduced FT by referring to would, could, and should counterfactuals as the basis for perceptions of unfair treatment. Specifically, would refers to a counterfactual contrast of outcomes, could indicates that a decision maker had been able to provide better outcomes than those actually obtained by the recipient, and should refers to the perception of a discrepancy between the decision-maker’s behavior and existing moral norms. A review of the literature shows how various studies have confirmed FT predictions. Finally, the chapter briefly touches on how a currently evolving perspective—deonance theory—has addressed some shortcomings of FT and gone beyond the justice context into the broader realm of behavioral ethics.