Q-ships, also known as Q-boats, Decoy Vessels, Special Service Ships, or Mystery Ships, were heavily armed merchant ships with concealed weaponry, designed to lure submarines into making surface attacks. This gave Q-ships the chance to open fire and sink them. The basic ethos of every Q-ship was to be a wolf in sheep's clothing.

They were used by the British Royal Navy (RN) during the First World War and by both the RN and the United States Navy during the Second World War (1939–1945), as a countermeasure against German U-boats and Japanese submarines.

Following the First Battle of the Atlantic, by 1915 Britain was in desperate need of a countermeasure against the U-boats that were strangling her sea-lanes. Convoys, which had proved effective in earlier times (and would again prove effective during the Second World War), were rejected by the resource-strapped Admiralty and the independent captains. Depth charges of the time were relatively primitive, and almost the only chance of sinking a submarine was by gunfire or by ramming while on the surface. The problem was luring the U-boat to the surface.

Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing, entertainment, public safety, and warfare. Historically, a "ship" was a vessel with sails rigged in a specific manner.

Ships and boats have developed alongside mankind. In armed conflict and in daily life they have become an integral part of modern commercial and military systems. Fishing boats are used by millions of fishermen throughout the world. Military forces operate vessels for combat and to transport and support forces ashore. Commercial vessels, nearly 35,000 in number, carried 7.4 billion tons of cargo in 2007.

Ships were key in history's great explorations and scientific and technological development. Navigators such as Zheng He spread such inventions as the compass and gunpowder. Ships have been used for such purposes as colonization and the slave trade, and have served scientific, cultural, and humanitarian needs. After the 16th century, new crops that had come from and to the Americas via the European seafarers significantly contributed to the world's population growth. Maritime transport has shaped the world's economy into today's energy-intensive pattern.