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.

    對城市社區來說，很少有比它的警察更為重要的機構了，但少有課題像歷史學家們對此了解得那樣少。 早期的學術興趣是在政治科學家和社會學家中發展起來的，他們一般只研究他們自己當代的問題而對過去的問題只是偶爾帶過。 甚至公眾似乎也僅僅在犯罪浪潮、明目張膽的貪污或特別的恐怖事件發生時才關心。 政黨的忠誠支持者和改革家們通常是從政治的前途來看待警察這個機構；而報紙和雜志-19世紀的傳播媒介-則著重活潑生動和驚人的事件。 可是城市社會總是把廣泛得可怕的責任交給警察。 他們不僅維持秩序、防止犯罪、保護生命財產，而且在過去還要救火、鎮壓罪惡、協助醫療服務、監督選舉、指揮交通、檢查建筑物、尋找逃學學生和搜捕逃犯。 除此以外，警察還被認為是公民自由和社會穩定的特別保護者。 自然，警察的表現并不盡如人意。 他們的記錄中有成功者，但多數是失敗的；有高效率的領導，但多半的人在職務上無能和不講信用。 專業警察部隊的概念在美國還是20世紀的產物；直到我們這個時代，一些城市才開始采取設立現代化部門的必要步驟。