Rounded, single-seeded stone fruits of the walnut tree, walnuts are a high-density source of nutrients, particularly proteins and essential fatty acids. Like other tree nuts, walnuts must be processed and stored properly. Grown for their seeds, the Persian or English Walnut and the Black Walnut are the two most common major species of walnuts.

The husk of the walnut, which is peeled away from the shell at harvest, and contain juice which will readily stain anything it comes in contact with. The husk juice has been used as a cloth dye.

The United States is the world's largest exporter of walnuts.