The history of Porto dates back to the 4th century, to the Roman occupation of the Iberian Peninsula. Celtic and Proto-Celtic ruins have been discovered in several areas, and their occupation has been dated to about 275 BC. During the Roman occupation, the city developed as an important commercial port, primarily in the trade between Olissipona (the modern Lisbon) and Bracara Augusta (the modern Braga).
Porto fell under the control of the Moors during the invasion of the Iberian Peninsula in AD 711. In 868, Vímara Peres, a Christian warlord from Gallaecia, and a vassal of the King of Asturias, Léon and Galicia, Alfonso III, was sent to reconquer and secure the lands from the Moors. This included the area from the Minho to the Douro River: the settlement of Portus Cale and the area that is today known as Vila Nova de Gaia. Portus Cale, later referred to as Portucale, was the origin for the modern name of Portugal. In 868 Count Vímara Peres established the First County of Portugal, or (Portuguese: Condado de Portucale), usually known as Condado Portucalense after reconquering the region north of Douro.
In 1387, Porto was the site of the marriage of John I of Portugal and Philippa of Lancaster, daughter of John of Gaunt; this symbolized a long-standing military alliance between Portugal and England. The Portuguese-English alliance,(see the Treaty of Windsor (1386)) is the world's oldest recorded military alliance.
In the 14th and the 15th centuries, Porto's shipyards contributed to the development of Portuguese shipbuilding. It was also from the port of Porto that, in 1415, Prince Henry the Navigator (son of John I of Portugal) embarked on the conquest of the Moorish port of Ceuta, in northern Morocco. This expedition by the King and his fleet, which counted amongst others Prince Henry, was followed by navigation and exploration along the western coast of Africa, initiating the Portuguese Age of Discovery. The nickname given to the people of Porto began in those days; Portuenses are to this day, colloquially, referred to as tripeiros (English: tripe peoples), referring to this period of history, when higher-quality cuts of meat were shipped from Porto with their sailors, while off-cuts and by-products, such as tripe, were left behind for the citizens of Porto: tripe remains a culturally important dish in modern day Porto.