Assesses and collects customs fees and penalties. Intercepts and seizes contraband, including narcotics and illegal drugs. Processes persons, carriers, cargo, and mail into and out of the United States. Administers navigation laws. Detects and apprehends violators of customs laws. Enforces export control laws. Cooperates with other federal agencies and foreign governments in suppressing illegal traffic in narcotics and pornography. Collects international trade statistics.

Historical functions of the Customs Service included administration of navigation aids and lighthouses, transferred to Lighthouse Board by act of August 31, 1852 (10 Stat. 119); and functions relating to the protection of seamen, and to revenue cutters, transferred to Revenue Marine Division (later Revenue Cutter Service) in 1871. The Bureau of Customs assumed responsibility for vessel documentation by EO 9083, February 28, 1942, which abolished the Bureau of Marine Inspection and Navigation.