This article is about the tree; for other meanings of hazel, see Hazel (disambiguation).

Hazels are a group of about ten species of trees and large shrubs that are native to the temperate northern hemisphere. The botanic name of the genus is Corylus, and it is placed in the family Corylaceae, though some botanists include this family within the Betulaceae.

The nuts obtained from the species Corylus avellana are the common edible hazelnuts. This large shrub is grown extensively for its nuts. Nuts are also harvested from some of the other species, including the filbert, from the Balkan species Corylus maxima.

Some species, such as the Turkish hazel are cultivated as ornamental trees. A number of ornamental garden forms of the common hazel are found, including forms with contorted stems, with purple leaves, and with weeping branches.

The species are grouped as follows:

• Nut surrounded by a soft, leafy involucre. Multi-stemmed, suckering shrubs to 12m tall.
  • Corylus americana - American hazel, from eastern North America
  • Corylus avellana - Common hazel, from Europe, North Africa and West Asia
  • Corylus cornuta - Beaked hazel, of eastern North America
  • Corylus maxima - Filbert, of southeastern Europe
  • Corylus sieboldiana - Japanese hazel, from northeastern Asia and Japan
• Nut surrounded by a stiff, somewhat spiny involucre. Large, single-stem trees to 30m tall.
  • Corylus colurna - Turkish hazel, from southeastern Europe and Asia Minor
  • Corylus jacquemontii - Jacquemont's hazel, from the Himalaya
  • Corylus chinensis - Chinese hazel, from West China
• Nut surrounded by a stiff, densely spiny involucre. Medium-size, single-stem trees to 15m tall.
  • Corylus ferox - Himalayan hazel, from the Himalaya.
  • Corylus tibetica - Tibetan hazel, from Tibet and southwest China.