The Douro valley was the first wine producing region to be controlled, anywhere in the world, when in 1756 the Marquis de Pombal by act of the Portuguese Government specified the area and production methods that were approved for Port production.

Firstly the region regarded as the Douro, where Port grapes are produced, is geographically demarcated, and is further split into 3 regions :
- the Baixo Corgo, which is nearest to the coast and has the lowest temperatures, produces bigger crops of lighter wines.
- the Cima Corgo, further inland, is the mainstay of grape production, producing over half of the required crop. The Cima Corgo is where you will find most of the famous quintas.
- the Douro Superiore, which almost reaches the border with Spain, is extremely arid and endures very high temperatures producing small quantities (only contributing around 5% of the total grape requirements) of extremely concentrated grapes.

Port like all other wine regions is tightly controlled by the Casa do Douro, who classify the vineyards into six grades, A to F, with A being the highest rated.

The Casa do Douro monitors and classifies vineyards into six grades, A to F, with A being the top grade. In order to achieve this grading the Casa do Douro has nine criteria that it assesses and awards points for.

Depending on how many points the vineyard is actually awarded it is classified as A to F, and classification of the vineyard regulates how much Port it is allowed to produce (in litres per 1,000 vines).

F-graded vineyards are not allowed to produce Port but they are allowed to produce normal un-fortified table wine.