Walnut (Juglans regia L.) is the most widespread tree nut in the world. There is a great diversity of genotypes differing in forestry, productivity, physical and chemical nut traits. Using a range of methodologies, from morphological markers to the most recent advances in genome analysis, many genetic studies of walnut have been conducted during the past years, including examination of diversity, determination of relationships within or among germplasm collections and populations, phylogenetic and origin elucidation, genetic map construction, and biotic or abiotic stress investigations. The genetic improvement of walnut has undergone great evolution. Some of them have been evaluated as promising and may serve as germplasm sources for breeding. Germplasm diversity is commonly evaluated with the help of morphological descriptors. This is usually the first step in classifying and describing germplasm and in studying heritability of traits for a new breeding program and selection of superior genotypes. The nutritional importance of the nut is related to the seed (kernel). It is a nutrientdense food mainly owing to its oil content, which can be extracted easily by screw pressing and consumed without refining. Walnut oil composition is dominated largely by unsaturated fatty acids. Phenolic compounds, present at high levels in the seed coat but poorly extracted with the oil, is extensively characterised and found to possess strong antioxidant properties. The oil extraction residue is rich in proteins and is employed in the formulation of various functional food products. Walnuts can nourish brain cells to improve human memory. Currently, there is an immense interest on identification of active constituents from walnut and screening those active compounds for pharmacological activities. Tree nut oils are primarily composed of triacylglycerols, but also contain diacylglycerols, monoacylglycerols, free fatty acids, and other minor components, including natural antioxidants and fat-soluble vitamins. The chemical composition of edible fats and oils largely determines their stability, quality, nutritional value, sensory properties, and potential health effects. Tree nuts, in many cases, provide rich sources of food lipids; up to 75% lipid on a weight basis. Generally, tree nuts are rich in monounsaturated fatty acids, predominantly oleic acid, but contain much lower amounts of polyunsaturated fatty acids, predominantly linoleic acid and small amounts of saturated lipids. In many parts of the world, tree nuts are cultivated for use as oil crops and are important sources of energy and essential dietary nutrients as well as phytochemicals. Plant breeding has always impacted food production and played a vital role in improving human nutrition. However, this has also increased uniformity within the world’s agricultural crops, contributing to increased genetic vulnerability to biotic and abiotic stresses. For these reasons, it is important to understand better the impacts of modern plant breeding on genetic diversity. In the same way, intelligent management of this diversity could be of valuable assistance to breeders.