Few institutions are more important to an urban community than its police, yet there are few subjects historians know so little about. Most of the early academic interests developed among political scientists and sociologists, who usually examined their own contemporary problems with only a nod toward the past. Even the public seemed concerned only during crime waves, periods of blatant corruption, or after a particularly grisly episode. Party regulars and reformers generally viewed the institution from a political perspective; newspapers and magazines - the nineteenth century's media - emphasized the vivid and spectacular. 

    Yet urban society has always vested a wide, indeed awesome, responsibility in its police. Not only were they to maintain order, prevent crime, and protect life and property, but historically they were also to fight fires, suppress vice, assist in health services, supervise elections, direct traffic, inspect buildings, and locate truants and runaways. In addition, it was assumed that the police were the special guardians of the citizens' liberties and the community's tranquillity. Of course, the performance never matched expectations. The record contains some success, but mostly failure; some effective leadership, but largely official incompetence and betrayal. The notion of a professional police force in America is a creation of the twentieth century; not until our own time have cities begun to take the steps necessary to produce modern departments.

    对城市社区来说，很少有比它的警察更为重要的机构了，但少有课题像历史学家们对此了解得那样少。 早期的学术兴趣是在政治科学家和社会学家中发展起来的，他们一般只研究他们自己当代的问题而对过去的问题只是偶尔带过。 甚至公众似乎也仅仅在犯罪浪潮、明目张胆的贪污或特别的恐怖事件发生时才关心。 政党的忠诚支持者和改革家们通常是从政治的前途来看待警察这个机构；而报纸和杂志-19世纪的传播媒介-则着重活泼生动和惊人的事件。 可是城市社会总是把广泛得可怕的责任交给警察。 他们不仅维持秩序、防止犯罪、保护生命财产，而且在过去还要救火、镇压罪恶、协助医疗服务、监督选举、指挥交通、检查建筑物、寻找逃学学生和搜捕逃犯。 除此以外，警察还被认为是公民自由和社会稳定的特别保护者。 自然，警察的表现并不尽如人意。 他们的记录中有成功者，但多数是失败的；有高效率的领导，但多半的人在职务上无能和不讲信用。 专业警察部队的概念在美国还是20世纪的产物；直到我们这个时代，一些城市才开始采取设立现代化部门的必要步骤。