A merchant vessel is a ship that transports cargo and passengers during peace time. During wars, the same ships are auxiliaries to the navies of their respective countries, and are called upon to deliver military personnel and materiel.Most countries of the world operate fleets of merchant ships. However, due to the high costs of operations, today these fleets are in many cases sailing under the flags of nations that specialize in providing manpower and services at favourable terms. Such flags are known as "flags of convenience". Currently, Liberia and Panama are particularly favoured. Ownership of the vessels can be by any country, however.The Greek-owned fleet is the largest in the world. Today, the Greek fleet accounts for some 16 per cent of the world s tonnage; this makes it currently the largest single international merchant fleet in the world, albeit not the largest in history.In English, "Merchant Navy" without further clarification is used to refer to the British Merchant Navy; the United States merchant fleet is known as the United States Merchant Marine.Merchant ships may be divided into several categories, according to their purpose and/or size.