Salt was a vital product in pre-industrial societies. Its value is associated with its preserving characteristics, which allowed foodstuffs and hides to be preserved for a relatively long time. This quality made it vital for the maintenance of pre-industrial activities like the fisheries. During the Middle Ages, most of the South Western European ports produced enough salt to respond to the needs of their urban and rural populations. With the growth of maritime activity and commercial expansion during the fourteenth and fifteenth centuries, salt became one of the most important bulk products in Europe, second only to grain.This article explores the surge of Aveiro and Setubal, two Portuguese ports specialized in the export of salt to the international markets and their contribution to the establishment, growth and development of an intricate set of commercial networks linking these ports to Amsterdam, Northern Europe and the Baltic. In the Portuguese context these ports were vital to the survival of the kingdom of Portugal, since most of the mid- and late seventeenth century diplomatic settlements were paid for by salt cargos and duties. For the Portuguese, salt was more than a bulk product. After 1640, it had become a hard currency.