Almost every nation has a merchant marine, a fleet of ships which are usually owned and registered in that nation and fly under its flag, but are separate from the military. The merchant marine carries goods and people, and is a vital part of the national economy. In addition, many countries use their merchant marine to supplement the military in times of war, with the merchant marine transporting goods and equipment to areas where it is needed. For this reason, the merchant marine is sometimes called the merchant navy, indicating the dual role of the merchant marine.

The people who staff the merchant marine are known as merchant marines, and while some have served in the military as well, most have chosen a career as merchant marines. The merchant marines are usually well trained, dedicated men and women who have ranks and procedures just like members of the military do. Many merchant marines have served during periods of war, and have been recognized and decorated for services to their home governments.

Merchant marines have a system of ranks and specialties which closely correlates to that of the military. Some merchant marines specialize in a particular field, such as navigation, communications, or heavy equipment operations. Others attend a merchant marine academy to begin careers as officers. Most merchant marines intend to remain in the merchant marine for the duration of their careers, and many companies provide benefits and incentives to encourage their staff to continue serving.

Most governments recognize the importance of the merchant marines to the economy and safety of the nation, and therefore supervise a Merchant Marine Academy. In the United States, the Merchant Marine Academy has been run by the government since 1943. Graduates of the academy have taken part in conflicts such as Vietnam and the Gulf War, in addition to serving as merchant marines on their home ships.

In general, to be considered a member of a nation's merchant marine, a ship must fly that nation's flag. Some ships may have foreign registration, but choose to fly under the flag of another nation for various reasons. The staff may be of mixed nationalities, although generally the officers represent the nation whose flag is flown. Depending on the country, some of the merchant marines may have military training as well as nautical training.

Merchant marines also rely on an extensive ground network, which includes shipyards, truck and train systems, and ground staff who manage the company. When marine insurance companies, lawyers, research companies, and a variety of other industries related to the merchant marine are added, it represents a sizable portion of the economy, with ships being a crucial part of the transportation system.