Between the arrival of transatlantic steamships in the 1830s and the popularization of air transport in the 1950s, shipping companies provided the only means of crossing the Atlantic, for both people as well as goods and mail. Often linked to governments by agreements, notably postal ones, they also had the important mission of representing national prestige, particularly in the United Kingdom, but also in France, Germany, and the United States. Their study thus makes it possible to determine what the promising migratory flows were during different periods, and how they were shared between the different companies. Ownership of these companies also became an issue that transcended borders, as in the case of the trust belonging to the financier John Pierpont Morgan, the International Mercantile Marine Company, which included British and American companies. A major concern from the viewpoint of states, their most prestigious ships became veritable ambassadors tasked with exporting the know-how and culture of their part of the world.