The ripening of grape berries is accompanied by a massive accumulation of soluble sugars, and by the synthesis and accumulation of a wide range of phenolic compounds and aroma precursors. These processes play major roles in the quality of the berries and wine. Sugars are accumulated in the vacuoles of flesh (mesocarp) cells, which account for 65 to 91 % of the fresh weight in a mature berry. Polyphenols accumulate in the skin and seeds, which represent respectively 6 to 20 %, and 2 to 6 % of the berry fresh weight, depending on the cultivars (Galet 1983). Although sugars and polyphenols do not accumulate in the same cells, evidence is growing for some control of polyphenol metabolism by sugars. The expression and activity of sugar transporters mediating sugar accumulation in the berries are also partially controlled by sugars.