The sense of smell is the key to enjoying wine: the olfactory examination considers the wine's odors as sensed through the nose.

Wine's molecules of smell consist of odorous vapors mainly in the form of volatile esters and aldehydes. When we sniff a wine, the aroma is passed up, through the olfactory cells and the lining in the upper part of the nasal cavity, to the olfactory bulb located at the base of the skull. This organ enables the brain to perceive not only odors inhaled through the nostrils, but also the wine's aromatic qualities conveyed to it from the palate by way of the nasal passage at the back of the mouth.

Experienced tasters learn to pick out the nuances of aroma and bouquet and associate them with certain types of wine.

The intensity of aroma is of relative importance since it is basically a measure of quantity. In a fine wine a rich bouquet is an attribute; but in a bad wine a strong odor is a negative factor. Persistence of aroma is an indication of quality, particularly in the lingering bouquet of a mature wine, but in a young, fruity wine it is not always an essential factor. The final test of quality is if the aroma of the wine being considered is expressed true to type. Such aromas depend on the grape variety and origins of the wine, on its age and how it has been aged or stored. There are three general categories of aroma to consider:

Primary aromas

These are odors intrinsic to the grape, usually sensed in young wines, often white, fermented at cool temperatures. Primary aromas are the source of so-called varietal character of certain wines, though the intensity and finesse of these grapey aromas vary from one variety to another.

Secondary aromas

These are the odors that derive from the alcoholic fermentation and, to some extent, from maturing in wood. They are sensed in young wines whose still aggressive fermentation odors are sometimes described as vinous or those that retain a certain freshness in fruit flavors over time.

Tertiary aromas

These are the distinctive odors, usually known as bouquet, that develop after alcoholic and malolactic fermentations are complete. They often derive in part from maturing in wood, though bouquets gain depth and complexity through aging in bottle.

The olfactory examination includes two stages: first it assesses the intensity, persistence and general quality of aromas, then it focuses on identifying nuances of aroma and associating them with the odors of other things, such as fruits, flowers, herbs and spices.

The intensity of aromas may vary from powerful or penetrating to light or scarcely perceptible. At the same time some odors linger in the nasal cavity - they have persistence - while others are fleeting, sensed only for an instant.

Tasters have somewhat different ways of detecting aromas. Some rely on a series of quick sniffs, others on long, gentle inhalations. Light swirling in the glass is enough to release the aromas of most wines. But be aware that some wines, as they evolve, reach inexpressive phases described as closed or dumb (tasters may attempt to free the odors of such wines by rapid rotating of the glass or even by holding one's hand over the rim and shaking it up and down.).

The olfactory examination must be limited to be effective. Prolonged smelling of the same odor reduces sensitivity to it, to the point that the olfactory system becomes saturated and can't detect it at all. That is why tasters often smell a wine only briefly before tasting it and then later smell it again. The bouquets of certain aged wines may continue to evolve for hours or even days.