Hazelnut (Corylus spp.) is a member of the Betulaceae family, and is widely distributed in temperate regions of the northern hemisphere. Cultivated varieties differ from each other in such characteristics as nut size, shape, kernel ratio, shell thickness, etc. Key target characteristics for breeding not only include high yield but also good morphological and vegetative traits as well as resistance to pests and diseases. This chapter discusses advances in the breeding of hazelnuts. Following an introduction, the author first presents a section on floral biology, pollination and compatibility in hazelnut. The chapter then includes information on the development of hazelnut varieties mainly relating to hazelnut breeding programmes in Turkey, Europe and also the United States (where the focus has been for breeding for resistance to Easter filbert blight). Then follows a section on the breeding of rootstocks referring to programmes in the United States, Turkey, Italy and Iran. A number of new key cultivars of hazelnut are then described.