Two chemical classes of flavonoids, the flavan-3-ols (catechins and proanthocyanidins) and the anthocyanins, are the natural antioxidants present at the highest concentration in red grape and wine. In the berry, the anthocyanins are localized in the skins, similarly to other highly bioactive phenolics of grape such as the resveratrols and the flavonols, while the flavan-3-ols are contained both in the skins and seeds. During winemaking, only a fraction of the grape flavonoids are selectively extracted into the wine, with a time course and a final yield strongly depending on the grape variety. The knowledge of the diverse and cultivar-specific characteristics of the grape is therefore critical to the appropriate design of the winemaking process. By means of a selective extraction method specifically designed to mimic the winemaking process, it was possible to analyze the "phenolic potential of red grape," thus obtaining quantitative information about amount and localization of the extractable flavonoids in the grape. Twenty-five high-quality red grape cultivars (V. vinifera) were studied, including 4 of the worldwide leading cultivars and 21 Italian cultivars with the highest reputation for the production of both young and aged premium red wines. The results clearly indicate that the grape variety plays a central role in determining both the absolute amount of the flavonoids, and the distribution between the berry skin and seeds of the flavan-3-ols. The very high biodiversity of the red grape cultivars in terms of flavonoids indicate a largely under-exploited opportunity to produce a range of diverse premium wines with optimized levels of natural antioxidants.