Red Port is blended together using a number of grape varieties. The precise identity of these, and the proportion each represents in the final blend, may not be known even to the winemaker. For some growers in the Douro region of Portugal, the ambiguity over what is in their fortified wines is not an attempt to protect some proprietary secret, but rather a matter of reality: they simply don't know for sure what they have growing in their vineyards.

Because Portugal's wine industry has adopted regulation and legally enforced controls in the past few decades, tradition still has a strong influence on how wines are made and vineyards are managed. Tradition, here, means co-planted varieties and field blends (wines made from whatever combination of varieties happens to occur in the source vineyard). So, while the New World wine regions were proudly leading the charge towards varietal winemaking, Portugal remained entrenched in age-old viticultural and winemaking techniques. As a result, the list of grape varieties sanctioned for use in Port wines is very long. There are roughly 80 permitted in Red Port and almost 50 in White Port.