The Merchant Marine encompasses the commercial uses of vessels and the people who operate them. Ships carry more than 90 percent of all goods imported to or exported from the United States. Although the vast majority of these are not operated by US citizens or the government, the international maritime community plays a significant role in American commerce. The Merchant Marine is not a branch of the military, but historically there have been close ties because of the key role supplies play during wartime.

Vessels that are operated commercially include tankers (which carry petroleum at all stages of the refining process and other liquid cargoes), container carriers (including dry liquid and refrigerated containers), dry bulk carriers (for grains and ores), roll-on/roll-off ships, also called “ro-ro” (for automobiles and other rolling equipment), cruise ships (passenger ships), harbor craft (tug boats and spill-response vessels) and specialty ships (cablelaying vessels, research vessels, etc.). While cruise ships may have hundreds of seamen on their crews to meet the needs of the passengers, most of the cargo ships operate with less than thirty people (even on long international voyages), and harbor tugs can operate with a crew of only two or three.

Most Americans are not aware of the significant role the Merchant Marine plays as the carrier of the world’s trade. Large commercial ships can only enter a limited number of ports, often located out of sight in industrialized areas of coastal cities. The rise of commercial air travel in the twentieth century also reduced awareness.