Depending on who you consult, there are between 14 and 18 species in this genus. They are deciduous trees and large shrubs found primarily in the temperate zones of the Northern Hemisphere.

One of the more often used hazelnuts used in the landscape is Harry Lauder's Walking stick, Corylus avellana 'Contorta'. Note: There was a Scottish vaudeville comedian named Sir Harry Lauder but I am not sure if there is a connection.