The Emergency Service Unit (ESU) is a police tactical unit within the New York City Police Department which provides specialized equipment, expertise and support to the various units within the police departments. These police forces do not have a traditional S.W.A.T. unit. The Emergency Service Unit qualifies in the role of a S.W.A.T. unit and much more. From auto accidents to building collapses to hostage situations, "ESU" officers are called on when the situation requires advanced equipment and expertise. The K9 Unit provides assistance during searches for missing persons, perpetrators, and evidence such as drugs and contraband.
When ESU was assigned to help the Organized Crime Control Bureau help arrest Richard Wheatley and his associates, Captain Chambliss called them off after they found out that Sergeant Ayanna Bell's wife at the time, Denise Bullock, was suing the NYPD. This left the OCCB without backup during the arrest. (OC: "Everybody Takes a Beating Sometime")
A group of criminals called the Robin Hood gang impersonated ESU officers to rob drug dealers. They were lead by Vaughn Davis, a police academy washout who kept in touch with some of his classmates who gave him access to some NYPD equipment. (OC: "Behind Blue Eyes", "Blaze Of Glory")