Chestnuts have been an integral part of the human diet for centuries, with nine commercially important species. Castanea sativa, commonly known as European or sweet chestnut, has unique sensory properties and high nutritional value, emphasizing its industrial relevance. Besides vitamins and minerals, chestnuts are rich in polyphenols, flavonoids, and tannins, contributing to their antioxidant, anti-inflammatory, and antimicrobial properties, and making them beneficial for preventing numerous chronic diseases. Chestnuts, especially valued for their edible nuts, impact both environmental and economic sectors, being traditionally consumed raw, roasted, and boiled. Nevertheless, innovative chestnut-based products have been developed to meet dietary needs, proposing alternatives for celiac disease and gluten intolerance and enhancing the versatility and commercial appeal of these nuts. Furthermore, the chestnut industry generates substantial amounts of by-products, namely shells, burs, and leaves. Recent advances highlight the benefits of valorizing these by-products into natural antioxidants, preservatives, and bioplastics sustained by the abundance of pro-healthy compounds. This chapter delves into the nutritional composition, phytochemical profile, and biological activities of chestnuts and their by-products, emphasizing their industrial applications and the importance of their sustainable management strategies for agro-waste.