Biology, the study of life, is important in understanding wine because wine is alive. Grapes are the fruit of living vines, and are themselves home to biological activity; wild yeast strains and the preferred wine-producing yeast, Saccharomyces cerevisiae, are found on the grape skins.

Grapes, and ultimately wine, are affected by living moulds, yeasts and bacteria. There are diseases that affect grapes and can kill the vine. For example, in 1863 Phylloxera (an American louse that kills grapevines) spread throughout Europe and destroyed many vineyards. Using biological methods European growers were able to preserve the European grapes by "grafting" (splicing) them to American roots, which were immune to the bug.

The warmth of the sun increases the sugar content of the grapes as they grow on the vine. During fermentation, this sugar provides food for the yeast (still on the skins, along with bacteria). Sometimes additional yeast is added, and the type and amount, as well as the temperature during fermentation will affect the time required for maturation and the final flavour. Living micro organisms can ruin the wine and so the fermentation must be controlled; a common method is to add sulphur dioxide (in legally defined quantities).

After the yeasts convert the sugar into alcohol, lactic acid bacteria convert malic acid to lactic acid and carbon dioxide in a process called "malolactic fermentation". The bacteria lower the acidity and add flavour compounds, ultimately changing the flavour of the wine.

Yeasts and bacteria in the wine can continue to change its flavour as it ages (in wooden barrels, stainless steel tanks, and even in the bottle). It is the living element in wine that can improve or ruin a wine's flavour with age.