This dissertation investigated a very specialised, highly sensitive and complex research area in policing known as undercover policing or covert policing operations. This is the first examination to be conducted in the Australasian policing context and the only known research to explore, empirically, undercover operatives' experiences of returning to mainstream police duties after completing their covert duties.----- There were two main research objectives in this dissertation. The first was to develop research methodology specific for use with undercover police officers. The second main objective was to conduct an empirical investigation to identify the psychological processes associated with the reintegration or re-assimilation of undercover police officers into mainstream policing environments. Social identity theory was applied to deconstruct undercover police officers' reintegration experiences.----- Given the closed and protected nature of covert policing, careful consideration to methodological and ethical issues required high priority in the development of research practices. Addressing these considerations in research practices protected the anonymity and security of those involved in the research. Tailoring research methods to suit the officers' circumstances and satisfy police managements' security concerns improved the practical application of research methods and research relations with police members and, therefore, the quality of the findings.----- In developing a research methodology for specific use with undercover police officers, a multi-method approach was adopted. Data triangulation with the use of a variety of data sources and methodological triangulation with the use of multiple methods and multiple indicators were employed. This technique proved constructive in creating a more holistic perspective of undercover policing and officers' experiences of re-assimilation.----- In theoretical terms, the major issue under investigation is of negotiating dual memberships or multiple identities. Three studies are reported. The first study is a field study, in which the researcher spent more than 18 months in the covert policing context, as a participant observer. Through field research, the researcher was able to learn about the Australasian covert policing context; obtain in-house police documents; define research issues and hypotheses; understand methodological considerations; identify a psychological theoretical framework; and examine &quotthe fit " between theory and the social dynamics of covert policing. Other benefits of becoming immersed in the working life of undercover police officers and the police organisation included understanding the ways of proceeding and the social and organisational structure that exists among covert personnel.----- The second study interviewed 20 former covert police personnel, from two police jurisdictions, who had been reintegrated for more than three years. The majority of officers found returning to mainstream police duties a difficult experience and two separate profiles of reintegration experiences emerged from the data. This study identified the presence of more than one police identity among former operatives. It found that some officers internalise aspects of the undercover policing norms and use these police norms to define aspects of the self both as a police officer and as an individual. That is, role-playing the undercover police persona became an extension of the officer as an individual and contributed positively to their personal self-worth. It was noted that the majority of officers expressed cognitive confusion over how to behave in the mainstream policing environment after covert duties had ceased.----- The other profile to emerge from the data was of officers who characterised their undercover policing experiences as being more integrated into their overall police persona. Officers interviewed in this study employed different identity decision-making strategies to restructure their police identities. In sum, this study found that the extent, to which the undercover and mainstream memberships were integrated cognitively, influenced officers' experiences of reintegration.----- The third study is a cross-sectional design using survey methods. Thirty-eight trainees, 31 currently operational and 38 former undercover operatives from four police jurisdictions took part in this study. A group of mainstream police officers matched according to former operatives' age, gender and years of policing experience was also included. This study found that police identities change over the phases of undercover police work and that changes in former operatives' mainstream police identity were a function of covert police work. Cross-sectional comparisons revealed that former operatives' undercover police identity had declined since covert policing, however, officers' mainstream police identity had not significantly increased. Failure to increase identification with mainstream police after undercover police work has ceased has a number of implications in terms of predicting re-assimilation. Operatives most likely to experience difficulties were those who resisted the mainstream police identification and reported difficult relations with their mainstream peers. Trends analysis revealed that despite the physical change, 'cognitive' re-assimilation actually commences in the second year of the operatives' reintegration. These exploratory analyses revealed that following return to the mainstream policing environment, identity stressors were mostly likely to be experienced in the first year of reintegration.----- To determine psychological adjustment since undercover police work, the person-environment fit was also investigated in the study. Operatives' current perceptions of working in the mainstream context were reported using a number of behavioural and organisational indicators. Overall, this study found that former operatives remain committed to their policing profession, however, those who experienced identity stress during the re-assimilation process were less satisfied with their current work duties and failed to find their work interesting, tended to perceive undercover duties as having been detrimental to their career, and expressed greater intentions to leave the service within 12 months of the survey. Overall former operatives' satisfaction and commitment levels were not significantly different from mainstream officers. Mainstream police, however, reported being under greater pressure and felt more overworked in the mainstream context than former operatives. In summary, these organisational indicators revealed that the difficulties of re-assimilation and intentions to leave the service are more related to the stress of modifying officers' police identity during this period than the workload characteristics of mainstream policing.----- Overall these studies demonstrated that the process of negotiating police identities is an important psychological dynamic present in undercover operatives' reintegration experiences. The identity stress experienced during this period was shown to have a number of organisational-behavioural consequences, such as problematic intergroup relations and greater intentions to leave the police service after undercover police work. Based on findings from this research a number of practical recommendations are made and suggestions for the direction of future research are outlined. Contributions to theory are also discussed.