This thesis examines the impact of politically motivated terrorism on the police service in Great Britain and Northern Ireland since 1969. During the period reviewed, changes to the police philosophy in the United Kingdom have been dramatic. An increasingly militarized role for the police reflects increased levels of violence in society. Manpower shortages resulted in the replacement of pro-active policing in the mid-1960's, with a re-active variety based on enhanced mobility. Terrorism is a manifestation of the determination of minority groups in society to draw attention to their cause. Their acts constitute the most serious crimes known to British law. Terrorism demands, it does not ask. It is this priority that terrorism creates on police resources, which produces clear evidence of its direct influence on police evolution during the past two decades. The existence of an Anti-Terrorist Squad, the creation of facilities at New Scotland Yard to collate terrorist related intelligence gathered nationally and internationally, are evidence of the specialization which now exists to counter this threat. In order to manage the changed philosophy behind the specialization, technological improvement and large increases in resources (including manpower), there is a trend towards a new type of top level police manager capable of integrating the enhanced services. This thesis examines and analyzes a number of incidents involving terrorists. It draws conclusions about the police response to those incidents and their influence on the day to day function of the police and takes account of the impact of governmental and international constraints and pressures. The Northern Ireland case study examines the circumstances which inspired the total reconstruction of the Royal Ulster Constabulary. The influence of the continued activities of parliamentary terrorist groups in the province, on the reconstruction process, is regarded as central to this study.