Hazelnuts are produced by hazel trees, which grow in temperate climates in many parts of the world, although they are native to Europe and Asia. The distinctive slightly bitter flavor of hazelnuts is suitable for use in savory and sweet dishes, although the nuts are most frequently used in desserts, especially paired with chocolate. Hazelnuts generally ripen in late August, which is the best time to find the flavorful nuts. Most grocery stores carry hazelnuts year round, often unshelled and raw. Hazelnuts are also sometimes sold preserved in their own oil.

The hazel tree is a fast growing deciduous shrub which produces catkins in the spring. The catkins mature into groups of as many as five nuts, often hidden under the serrated green leaves. Because hazel is fast growing and easy to shape, it has a long history of use in hedging, especially in England. Left alone, hazel can reach a height of five feet (15 meters). The shrub also provides habitat to numerous animals and birds, as well as serving as a source of food for butterflies.

The alternate name of the hazelnut is filbert. The exact reason for this common name is unclear, although the nuts do tend to mature around the time of the feast day of St. Philbert. Some people distinguish between hazelnuts and filberts, arguing that filberts are actually a different type of nut, although the two are related. People who differentiate between the two believe that filberts have slightly longer shells. When they are shelled, however, the two nuts look identical.

The shell of a hazelnut is brown, glossy, and roughly ovoid. Once shelled, the hazelnut still has a bitter dark brown skin, which many people remove before cooking the nuts. The flesh of hazelnuts is white, and slightly sweet when the bitter skin is not present. Many cooks toast hazelnuts before using them to enhance their mild flavor. The nuts appear ground with chocolate to make spreads, mixed in with stuffings, in hazelnut torte, and on a variety of other desserts. They can also be pressed to yield a dark, flavorful oil.

To select the best hazelnuts, look for plump specimens without any sign of shriveling, and plan on using them within one month or freezing them. If the shells of the hazelnuts are still on, look for smooth, glossy shells with no signs of cracks or holes, and shake them. The nuts should not rattle in the shell, as this indicates that they have lost moisture.