Sweetness is a taste sensation brought about by the presence of sugar. The types of wine that have enough residual sugars to release a sensation of sweetness are raisin wines, botrytised wines, late harvest wines, or wines made from harvesting frozen grapes. Fortified wines and spumantes are a case apart. Sugar is present in ripe grapes (fructose and glucose), but during fermentation the sugar is converted into alcohol by the action of yeasts. On the contrary, it often contains none, or only a marginal amount, and has a dry taste. For a wine to be sweet, there must therefore be enough sugar left at the end of fermentation to allow us to perceive the sensation of sweetness. 

To obtain a sweet wine, except in the case of spumante and fortified wines, the original grapes must be particularly rich in sugar. This is the case with the following types: Late harvest wines; Botrytised wines: Some of the world's most famous botrytised wines include Tokaji, named after the region of Hungary in which it is produced, Trockenbeerenauslese, produced in parts of Austria and Germany, and Sauternes produced in the Bordeaux area of France; Icewine and Eiswein; Raisin wines: a way of concentrating sugar in the grapes is to let them dry out after the harvest. On the island of Pantelleria, for example, Muscat grapes are left out to dry in the sun and in a few days they dehydrate to the point of producing a sweet wine with a heady aromatic fragrance: the famous Passito di Pantelleria. The same applies to Malvasia delle Lipari produced on the Aeolian Islands, or Greco di Bianco which hails from the south of Calabria. In other regions, however, the grape can be dried indoors, as is the case for famous raisin wines such as Recioto della Valpolicella in the province of Verona, Recioto di Soave and Gambellara in the province of Vicenza or Vin Santo Toscano.