The role of witnesses to mistreatment in the workplace is a relatively unexplored domain of the aggression literature. This thesis project integrates deontic justice (Folger, 2001; 2012) and scope of justice (Mitchell, Vogel, & Folger, 2012) theories in order to test a theoretical model of witness intervention behaviours in response to incivility. These two studies examine the effect of target competence on third-party witness behaviours towards both the target and perpetrator of incivility. In Study 1, the main effects of perceived target competence on witness behaviour are investigated through the use of confederate actors within a live laboratory study. In Study 2, a more comprehensive test of the theoretical model investigates the indirect effects through which target competence influences witness responses. Results indicate that a target’s perceived level of competence affects the extent to which a witness helps the target, confronts the perpetrator, and remains passive.