This thesis attempts to produce several contributions to police legitimacy. It consists of various moving yet intertwining parts. First, to capitalise on the growing body of evidence on the effects of 'procedural justice training' to police officers we conduct a systematic review to trace, evaluate, and synthesise all the results of published procedural justice training tests. Nine rigorous experiments have met our eligibility criteria. Overall, the results suggest that police officers exposed to training programmes on procedural justice perform better than unexposed officers, on multiple outcomes. However, when subgrouping the effects based on different parameters, we show varying and at times non-significant effects , particularly with classroom- based training approaches and mixed components- based style of training on frontline police officers and recruits. We highlight areas that are presently short on evidence, as a guide for future studies in this area. Second, we explore the theoretical relationship between training on procedural justice on both self-legitimacy as well as audience legitimacy as outcomes. The degree to which police officers maintain higher levels of self-legitimacy is hypothesised to effect how they conduct themselves which, in turn, translates into improved audience legitimacy. We therefore test the effect of a bespoke training package aimed at stimulating an officer's inner conversations and promoting their sense of self or self-legitimacy as well as audience legitimacy scores. Frontline police officers working within counterterrorism settings were randomly assigned to training or control conditions within a large-scale cluster randomised controlled trial. The results suggest differing results in terms of both measures. We show that the training has no statistically significant effects on the degree to which officers retain the belief in their moral right to be bestowed with the powers vested in them to effectively discharge the performance of their duties. However, the training leads to interactions that are perceived by suspects as more legitimate. We conclude that through training, interactions between police officers and members of the public are perceived as fairer. Scholars and practitioners interested in audience legitimacy should therefore concentrate on legitimacy training in an attempt to improve policing.