They may be broadly classified as inshore patrol vessels (IPVs) or offshore patrol vessels (OPVs). They are warships typically smaller in size than a corvette and can include fast attack craft, torpedo boats and missile boats, although some are as large as a frigate. The offshore patrol vessels are usually the smallest ship in a navy's fleet that is large and seaworthy enough to patrol off-shore in the open ocean. Smaller inshore patrol vessels also known as coastal patrol craft or coastal patrol boats. Riverine patrol craft are used for patrol river area. In larger militaries, such as in the United States military, offshore patrol vessels usually serve in the coast guard, but many smaller nations' navies operate these type of ships.
During both World Wars in order to rapidly build up numbers, all sides created auxiliary patrol boats by arming motorboats and seagoing fishing trawlers with machine guns and obsolescent naval weapons. Some modern patrol vessels are still based on fishing and leisure boats. Seagoing patrol boats are typically around 30 m (100 ft) in length and usually carry a single medium caliber artillery gun as main armament, and a variety of lighter secondary armament such as machine guns or a close-in weapon system. Depending on role, vessels in this class may also have more sophisticated sensors and fire control systems that would enable them to carry torpedoes, anti-ship and surface-to-air missiles.
Their small size and relatively low cost make them one of the most common type of warship in the world. Almost all navies operate at least a few offshore patrol vessels, especially those with only "green water" capabilities. They are useful in smaller seas such as the North Sea as well as in open oceans. Similar vessels for exclusively military duties include torpedo boats and missile boats. The United States Navy operated the Pegasus class of armed hydrofoils for years, in a patrol boat role. During the Vietnam war, the US Navy ordered 193 Fast Patrol Boats (FPV) from Swiftships, for brown water patrol and special operations. The River Patrol Boat (PBR, sometimes called "Riverine" and "Pibber") is a U.S. design of small patrol boat type designed to patrol waters of large rivers.
35m Fast Patrol Vessels- USA -built by Swiftships, Commissioned in 2021 
The Bangladesh Navy classified its medium size patrol ships as large patrol craft (LPC) which are armed with either anti-ship missiles or torpedoes. Those ships typically have heavier armaments but less range than OPVs.
Harbour security boat (PBI) – four newly built 80-ton class harbour security / patrol boats, and more are planned in order to take over the port security / patrol duties currently performed by the obsolete Shantou, Beihai, Huangpu, and Yulin-class gunboats, which are increasingly being converted to inshore surveying boats and range support boats
Cyclone-class patrol ship - (2000-2011) - four boats were transferred to the Coast Guard for a temporary loan, three (2004-2011) have since been returned, while the fourth (2000-2004) was donated to the Philippine Navy
^"Bahrain Naval Forces Commisioned New Patrol Vessels". Naval Post. 14 February 2021. Retrieved 15 April 2022.
^"Bangladesh and Asia's Maritime Balance". Center for International Maritime Security. 7 December 2015. Retrieved 6 December 2020.
^"Tender specifications for construction of 02 X Large Patrol Craft (LPC) in local shipyard for BN" (PDF). Directorate General of Defence Purchase. Retrieved 6 December 2020.
^"BDR-Shahjalal – Patrol Vessel – Details and current position IMO 8106472 MMSI 0 | Vessels | VesselFinder". www.vesselfinder.com. Retrieved 2017-04-23.
^"Brazil getting armored vehicles, boats". Space War. Space Media Network. UPI. 10 August 2012. Retrieved 19 August 2017. The first-of-class Amazonas was constructed at BAE Systems' Portsmouth facility.
^"Offshore Patrol Vessels". BAE Systems. n.d. Archived from the original on 17 January 2013. We are supplying three Ocean Patrol Vessels and ancillary support services to the Brazilian Navy, as well as a manufacturing licence to enable further vessels of the same class to be constructed in Brazil. P120 Amazonas, P121 Apa and P122 Araguari