The police system involves more than 20,000 distinct law enforcement agencies whose differences are generated mainly by the jurisdictional authority or boundaries under which they operate and the specific laws they are empowered to enforce, The United States is served by a multiple layered network of police jurisdictions that include town, city, county, and State police as well as Federal law enforcement agencies. The main structural elements of American policing emerged in the middle of the 19th century. Police officers' powers of arrest are restricted to the geographical boundaries of their employers. Law enforcement agencies have many formal and informal programs of cooperation and training, however. Meetings, seminars, and conferences provide excellent opportunities for sharing ideas and new programs and enhance the effectiveness of police throughout the nation. Photographs and 8 references.