All species are deciduous trees, 10–40 metres (33–131 ft) tall, with pinnate leaves 200–900 millimetres (7.9–35.4 in), with 5–25 leaflets; the shoots have chambered pith, a character shared with the wingnuts (Pterocarya), but not the hickories (Carya) in the same family.

The common name walnut derives from Old English wealhhnutu, literally 'foreign nut' (from wealh 'foreign' + hnutu 'nut'),[2] because it was introduced from Gaul and Italy.

A key trait for more northerly latitudes of North America and Europe is phenology, with ‘late flushing’ being particularly important to avoid frost damage in spring.

The male cylindrical catkins are developed from leafless shoots from the past year; they are about 10 cm (3.9 in) in length and have a large number of little flowers.

In Jammu, it is used widely as a prasad (offering) to Mother Goddess Vaisnav Devi and, generally, as a dry food in the season of festivals such as Diwali.

In the section Juglans, J. regia nut oil was found to contain from 10% to 11% linolenate, 6% to 7% palmitate, and a maximum concentration of linoleate (62% to 68%).

In the section Cardiocaryon, the nut oils of J. ailantifolia and J. mandshurica were reported to contain (respectively) 7% and 5% of linolenate, 2% of palmitate, and maximum concentrations of 74% and 79% linoleate.

Within the section Rhysocaryon, the nut oils of the U.S. native black walnuts J. microcarpa and J. nigra were reported to contain (respectively) 7% and 3% linolenate, 4% and 3% palmitate, and 70% and 69% linoleate.

Walnut wood has been the timber of choice for gun makers for centuries, including the Gewehr 98 and Lee–Enfield rifles of the First World War.

Walnut burl veneer is one of the most valuable and highly prized by cabinet makers and prestige car manufacturers.

The wood of the butternut and related Asian species is of much lower value, softer, coarser, less strong and heavy, and paler in colour.

In North America, forestry research has been undertaken, mostly on J. nigra, aiming to improve the quality of planting stock and markets.

[20] As early as the 2nd century CE, shells and kernels of the edible walnut were used to make a dye solution in the Levant.

[23][24] However, different walnut species vary in the amount of juglone they release from the roots and fallen leaves - J. nigra, in particular, is known for its toxicity, both to plants and horses.

In California (US) and Switzerland, crows have been witnessed taking walnuts into their beaks, flying up to 60 feet or so in the air, and dropping them to the ground to crack the shells and eat the nut inside.

The nuts are edible, and though they are often used in expensive baked goods, the Persian walnut is preferred for everyday use because it is easier to extract the nutmeat.

The Hinds' black walnut (J. hindsii) is native to northern California, where it has been widely used commercially as a rootstock for J. regia trees.

The Japanese walnut (J. ailantifolia) is similar to butternut, distinguished by the larger leaves up to 90 cm long, and round (not oval) nuts.

Its leaves are 40–60 cm long, the fruits are oval, the shell has very tall, very slender ridges, and the kernel is especially high in fat.

A study[29] of sequenced nuclear DNA from the external transcribed spacer (ETS) of ribosomal DNA (rDNA), the internal transcribed spacer (ITS) of rDNA, and the second intron of the LEAFY gene taken from at least one individual of most of the species of Juglans has supported several conclusions: The paper presenting these results did not publish any new names for the subdivisions of sect.

[30] The paleontological history of Juglans regia in Europe shows signs of a post-Ice-Age re-expansíon from refugia in the southeast, much influenced by people carrying walnut nuts about after the numbers of humans had been much increased by the start of agriculture.