The Cook County Sheriff's Office is a local law enforcement agency that serves Cook County, Illinois. It is the second largest Sheriff's department in the United States with over 6,900 members. It is headed by the Sheriff of Cook County, currently Thomas Dart. Due to its size, the Cook County Sheriff's Office divides its operations by task into 8 departments, the most recognizable of which is the Cook County Sheriff's Court Services Department. The much smaller Cook County Sheriff's Police Department provides traditional police services in unincorporated Cook County while the Department of Corrections staffs the Cook County Department of Corrections.
All Cook County Sheriff's Police Officers are Cook County Sheriff's deputies, but not all Cook County Sheriff's deputies are Sheriff's Police Officers. Police Officer is a job function and title within the Cook County Sheriff's Police Department. It should be noted that all Cook County Sheriff's Deputies have police powers regardless of their particular job function or title. Like other sheriffs' departments in Illinois, the Sheriff's Police can provide all traditional law-enforcement functions, including county-wide patrol and investigations irrespective of municipal boundaries, even in the city of Chicago, but has traditionally limited its police patrol functions to unincorporated areas of the county because unincorporated areas are the primary jurisdiction of a Sheriff's Department in Illinois.
The Sheriff's Police patrol services are often not required in incorporated cities because the cities such as Chicago have established their own police departments. The 500-600 member Sheriff's Police Department would not have the personnel necessary to supply full police services to all incorporated areas in Cook County especially in a municipality such as Chicago. Sheriff's deputies, outside the Sheriff's Police, provide the other services of the sheriff, such as guarding the various courthouses in Cook County, running and guarding the 9,800-detainee Cook County Jail, and overseeing other offender rehabilitation programs.