This book aims to describe the evolution of the structures devoted to maintaining security in ancient Rome through six major chronological scansions: the origins, the development, the crisis, the Principate, the Late Antiquity and the Justinian's Reign. The first chapter is devoted to the origins. It highlights how the characteristics of the city genesis lead to a community in which family, social, political and religious ties operate, and, in the absence of a modern police force, give life to forms of self-regulation. This real "system" is run by the rex, his auxiliaries, quaestores paricidii and duumviri perduellionis, and the by the pater familias who exercises his powers of coercion over his wife, sons, daughters, servi, liberti and clientes. Finally, each citizen is recognized guardian of himself. Chapters II and III are devoted to the development. In the first one some elements of the Roman police system are taken into account: the importance of family and client ties, the role of priests and the quality of Rome as inermis. Through the coercive powers of the consules and tribuni plebis, it is emphasized how and under what circumstances police functions were used for the defense of the aristocratic republic, on the one hand, and the plebeian community on the other. Chapter III expounds the activities of public security of some magistrates who, in correspondence with the growth of the city and the progressive complication of social and economic relationships, were involved in that matter, but always within a self-regulating system. These are the quaestores, the aediles and the tresviri, both nocturni and capitales. Chapter IV illustrates how the crisis of the republic dissolved the self-regulatory system described in the previous chapters and focuses on the role of extraordinary investigations, permanent courts and senatus consultum ultimum. The creation of personal security forces by individual politicians is also described highlighting elements of novelty and continuance, till the militarization of public order intervened in the mid-first century. B.C. A special section discusses the fight against piracy, in relation to which the creation of extraordinary military commands even has a constitutional significance, and where forms of imperium that anticipate Augustus' s one are experienced. Chapters V and VI are devoted to Securitas Augusti, that is, to the age of the Principate, and to the innovations made both in Rome and the provinces. In the first case, the author tries to understand how, through which solutions and the exercise of which functions, a city security system is reached, a system in which the republican magistracies do not operate anymore and their functions are performed by the prince officials and their soldiers organized into cohortes urbanae, vigilum and praetoriae. In the provinces, however, the creation of a police system that considers the governor to be at the head of public security and law enforcement is witnessed. Actions against the brigands, the creation and evolution of a system for the daily supervision of the land, stocks, prisons, fight against what remained of piracy, the role of the cities and their magistrates are described. Chapters VII and VIII are devoted to the age of Late Antiquity. The first one describes the situation of the imperial capitals, so that of Rome, even if it is no longer the imperial seat, and of Constantinople. Ravenna and Antioch are also considered, respectively for the West and the East. In this period the idea of self-regulation is still alive as it is implied in the social hierarchy relationships replicated in different forms. However, attempts to put it back into practice fail completely because in the end it could operate effectively only in an urban and republican context that has been long deceased. Chapter VIII discusses how the economic and military crisis and the late antiquity reforms, especially the separation between the civil and military powers, influenced security management. It is the period of the progressive centralization and the correlated reduction of municipal autonomies. A specific attention is given to some local entities, in order to illustrate the operation of the provincial security system in the Late Antiquity. A final chapter is devoted to the reforms made by Justinian in Constantinople and in the provinces. The Justinian' s Reign is taken into account as the term of this research, not only for its value in the delimitation of the Antiquity, but because of the specific circumstances related to his reforms. In all this development there was the idea of the need for an immediate and quick police repression in front of particularly dangerous and evident threats. This is an attitude, an almost cultural one, that coexists with the great results of social and legal progress represented by the achievements following the patrician-plebeian struggles or with the gradual limitation of the coercive powers of the supreme magistrates, but that constantly reappears even in those from whom we expect less than others behaviors that cannot be shared nowadays. During the Principate and in the Late Antiquity, this trend is found in different cases, for example in facultas opprimendi, or in the pressing need to establish self-defense forces. It is also evident in terms of jurisdiction through a series of rules restricting the right to appeal, the root of which is in the need to restore a broken order as soon as possible.