A police department is the branch of government concerned with enforcement of law and prevention of crime. The term police department refers to the central organization that takes care of the functioning of numerous sub agencies under it. In a vast and extensive country like the US, each state has a separate police department that takes care of maintaining the law and order. The police department plays a pivotal role in the smooth functioning of the law and order machinery in the respective state. The United States of America has a number of police departments that function towards preserving peace and security.

The New York City Police Department (NYPD) is the largest law enforcement agency in the US. It is mainly responsible for law enforcement and inquiry within The Five Boroughs of New York City. The New York City Police Commissioner heads the NYPD. The Police Commissioner assigns a number of Deputy and Assistant Commissioners. The Department is split up into 10 bureaus. This is further sub divided into sections, departments and units, and into boroughs, precincts and squads. The Bureau Chief commands each Bureau. There are also a number of specialized units that are not part of any of the Bureaus and report to the Chief of the Department or a Deputy Commissioner.

The Los Angeles Police Department commonly known as the LAPD is the police department of the City of Los Angeles, California. It enjoys the status of being of being the third largest law enforcement agencies in the US. Its work force includes over 9,000 officers and 3,000 civilian staff, covering an area of 467 miles with a population of around 3.4 million. LAPD has had a rich and contentious history.

The LAPD and the NYPD are two of the most efficient police departments in the US. Other forces in various states have their departments modeled on similar lines. The core basis of operandi and functioning are almost the same for all police departments in the US.