Plants in the genus produce citrus fruits, including important crops such as oranges, mandarins, lemons, grapefruits, pomelos, and limes.

With a propensity to hybridize between species, making their taxonomy complicated, there are numerous varieties encompassing a wide range of appearance and fruit flavors.

[4] A DNA study published in Nature in 2018 concludes that the genus Citrus evolved in the foothills of the Himalayas, in the area of Assam (India), western Yunnan (China), and northern Myanmar.

[7] A genomic, phylogenic, and biogeographical analysis by Wu et al. (2018) has shown that the center of origin of the genus Citrus is likely the southeast foothills of the Himalayas, in a region stretching from eastern Assam, northern Myanmar, to western Yunnan.

[7][6] This was followed by the spread of citrus species into Taiwan and Japan in the Early Pliocene (5.33 to 3.6 mya), resulting in the tachibana orange (C. tachibana); and beyond the Wallace Line into Papua New Guinea and Australia during the Early Pleistocene (2.5 million to 800,000 years ago), where further speciation events created in the Australian limes.

[10] In China, fossil leaf specimens of †Citrus linczangensis have been collected from late Miocene coal-bearing strata of the Bangmai Formation in Yunnan province.

Natural and cultivated citrus hybrids include commercially important fruit such as oranges, grapefruit, lemons, limes, and some tangerines.

[15][16] A recent taxonomy reincorporates the trifoliate orange (Poncirus) into an enlarged Citrus, but recognizes that many botanists still follow Swingle in splitting it off.

[21][22] In cooler parts of Europe, citrus fruit was grown in orangeries starting in the 17th century; many were as much status symbols as functional agricultural structures.

[24] Citrus plants are large shrubs or small to moderate-sized trees, reaching 5–15 m (16–49 ft) tall, with spiny shoots and alternately arranged evergreen leaves with an entire margin.

[30] Citrus fruits are diverse in size and shape, as well as in color and flavor, reflecting their biochemistry;[31][32] for instance, grapefruit is made bitter-tasting by a flavanone, naringin.

Most commercial citrus cultivation uses trees produced by grafting the desired fruiting cultivars onto rootstocks selected for disease resistance and hardiness.

[39] According to the United Nations Conference on Trade and Development, citrus production grew during the early 21st century mainly by the increase in cultivation areas, improvements in transportation and packaging, rising incomes and consumer preference for healthy foods.

[37] In 2019–20, world production of oranges was estimated to be 47.5 million tonnes, led by Brazil, Mexico, the European Union, and China as the largest producers.

[49] The condition is often caused by an excessively high pH (alkaline soil), which prevents the plant from absorbing nutrients such as iron, magnesium, and zinc needed to produce chlorophyll.

[52][53] In humans, some of these act as strong photosensitizers when applied topically to the skin, while others interact with medications when taken orally in the grapefruit juice effect.

[52] Due to the photosensitizing effects of certain furanocoumarins, some Citrus species cause phytophotodermatitis,[54] a potentially severe skin inflammation resulting from contact with a light-sensitizing botanical agent followed by exposure to ultraviolet light.

[63] The whole of the bitter orange (and sometimes other citrus fruits) including the peel with its essential oils is cooked with sugar to make marmalade.

[67] A wall painting in the tomb of Nakht in 15th century BC Egypt depicts a woman in a festival, holding a lemon.

[67] Citrus fruits "were the clear status symbols of the nobility in the ancient Mediterranean", according to the paleoethnobotanist Dafna Langgut.

[68] In Louisa May Alcott's 1868 novel Little Women, the character Amy March states that "It's nothing but limes now, for everyone is sucking them in their desks in schooltime, and trading them off for pencils, bead rings, paper dolls, or something else… If one girl likes another, she gives her a lime; if she’s mad with her, she eats one before her face, and doesn’t offer even a suck.