Since Gaston Glock first introduced his tenifer-coated polymer framed handgun in the 1980s, military and law enforcement agencies around the world have taken to it for the same reason that makes it a popular civilian handgun. The Glock is easy to use, accurate with a natural point of aim, lightweight, nearly unbreakable (although I have seen a large police K9 eat one), and reliable. In fact, in a recent survey of police officers, more than 70% confessed to owning a Glock as a personal firearm.
Let us look at who uses Glocks in the US
(Not a Glock user)
The .40 S&W chambered Glock 22 is one of the most popular handguns issued with law enforcement agencies across the country. The Los Angeles Police Department (LAPD) and all LA City affiliated police depts. (LAXPD, GSPD, PORT PD) issue them as does the Baltimore Police Department, Minneapolis/St. Paul PD, Louisiana State Police (along with no less than ten other state police agencies), and Miami PD among others. The Glock 22 and the smaller 23 compact is a favorite sidearm of the United States Department of Justice and many federal agencies, including the FBI, DEA, US Marshals Service, IRS CID, and others issue the polymer-framed weapon.
Some departments still are beholden to the 9x19mm Parabellum round for one reason or another. With certain loadings, it is less likely to penetrate in urban environments, its practice loadings are less expensive than comparable .40S&W and .45ACP rounds, and when matched with a modern JHP are extremely effective. Departments that still issue the Glock 17/19 include Virginia Beach PD, Albuquerque PD, and the Indiana State Police. The massive 35,000-member New York Police Department equip their force exclusively with Glock 19s modified with a special NY-1/NY-2 trigger to their specifications.
Mississippi State Police including MHP, MDWFP, MBI, MBN, and the State Gaming Commission carry the .45ACP Glock 21. This hard-hitting .45 seems to be very popular with rural departments, especially for county sheriffs and such well-known county agencies as the Johnston County, NC Sheriff Office, Muscogee County, GA, Sheriff Office and the Clay County, FL, Sheriffs. This if further seen in that the Anchorage Alaska PD issues their officers the Glock 21.
Even rare caliber Glocks are popular with law enforcement. A number of state law enforcement agencies including the Pennsylvania State Police, SCHP, Florida State Police/Highway Patrol and the New York State Police carry 45GAP chambered Glock 37s. Chicago Transit Police issue the Glock 32, the Pennsylvania Game Commission issue the Glock 31 in .357SIG,
In short, if you know a cop, odds are he knows a Glock.