Hazelnuts (Corylus avellana L.) are one of the four largest nuts in the world and is widely used in food processing (e.g. chocolate, confectionery) due to its unique flavour (some volatiles) and high nutritional value. With the widespread cultivation of hazelnuts, a large number are produced (hazelnut skins, hazelnut shells, hazelnut husks, hazelnut leaves). In recent years, a large number of studies have been conducted on hazelnut kernels and their by-products, which have been found to be rich in phytochemicals and possess a variety of biological activities. In this chapter, the special metabolites and biological activities of hazelnut kernel and its by-products are reviewed, which mainly include fatty acids, proteins, phenols, dietary fibre, and other metabolites with antioxidant, anti-inflammatory, anti-glycation, lipid-lowering, and antitumor effects. In particular, the paclitaxel substance found in hazelnut shells and husks can effectively inhibit the proliferation of cancerous cells and may be a good source of paclitaxel except in the case of yew. A comprehensive study of hazelnuts and their by-product metabolites makes them a source of compounds beneficial to human health. Meanwhile, it can also turn waste into treasure, reduce resource waste and environmental pollution, and improve the economic benefits of hazelnut cultivation.