Eucalyptus /ˌjuːkəˈlɪptəs/ L’Héritier 1789 is a diverse genus of flowering trees and shrubs (including a distinct group with a multiple-stem mallee growth habit) in the myrtle family, Myrtaceae. Members of the genus dominate the tree flora of Australia, and include Eucalyptus regnans, the tallest known flowering plant on Earth. There are more than 700 species of eucalyptus and most are native to Australia; a very small number are found in adjacent areas of New Guinea and Indonesia. One species, Eucalyptus deglupta, ranges as far north as the Philippines. Of the 15-species found outside Australia, just nine are exclusively non-Australian. Species of eucalyptus are cultivated widely in the tropical and temperate world, including the Americas, Europe, Africa, the Mediterranean Basin, the Middle East, China, and the Indian subcontinent. However, the range over which many eucalypts can be planted in the temperate zone is constrained by their limited cold tolerance. Australia is covered by 92,000,000 hectares (227,336,951 acres) of eucalypt forest, comprising three quarters of the area covered by native forest.

Some eucalyptus species have attracted attention from horticulturists, global development researchers, and environmentalists because of desirable traits such as being fast-growing sources of wood, producing oil that can be used for cleaning and as a natural insecticide, or an ability to be used to drain swamps and thereby reduce the risk of malaria. Eucalyptus oil finds many uses like in fuels, fragrances, insect repellences and antimicrobial activity. Eucalyptus trees show allelopathic effects; they release compounds which inhibit other plant species from growing nearby.

On warm days, eucalyptus forests are sometimes shrouded in a smog-like mist of vaporized volatile organic compounds (terpenoids); the Australian Blue Mountains take their name from the haze.

Eucalyptus was introduced from Australia to the rest of the world following the Cook expedition in 1770. Collected by Sir Joseph Banks, botanist on the expedition, it was subsequently introduced to many parts of the world, notably California, Brazil,. On the order of 250 species are under cultivation in California. In Portugal and also Spain, eucalypts have been planted in plantations for the production of pulpwood. Eucalyptus are the basis for several industries, such as sawmilling, pulp, charcoal and others. Eucalypts have many uses which have made them economically important trees, and have become a cash crop in poor areas such as Timbuktu, Africa and the Peruvian Andes, despite concerns that the trees are invasive in some countries like South Africa. Best-known are perhaps the varieties karri and yellow box. Due to their fast growth, the foremost benefit of these trees is their wood. They can be chopped off at the root and grow back again. They provide many desirable characteristics for use as ornament, timber, firewood and pulpwood. It is also used in a number of industries, from fence posts and charcoal to cellulose extraction for biofuels. Fast growth also makes eucalypts suitable as windbreaks and to reduce erosion.

Eucalypts draw a tremendous amount of water from the soil through the process of transpiration. They have been planted (or re-planted) in some places to lower the water table and reduce soil salination. Eucalypts have also been used as a way of reducing malaria by draining the soil in Algeria, Lebanon, Sicily, elsewhere in Europe, in Caucasus (Western Georgia), and California. Drainage removes swamps which provide a habitat for mosquito larvae. This drainage is not limited to the soil surface, because the eucalyptus roots are up to 2.5 m (8.2 ft.) in length and can, depending on the location, even reach the phreatic zone.

Eucalyptus oil is readily steam distilled from the leaves and can be used for cleaning and as an industrial solvent, as an antiseptic, for deodorizing, and in very small quantities in food supplements, especially sweets, cough drops, toothpaste and decongestants. Eucalyptus globulus is the principal source of eucalyptus oil worldwide. The nectar of some eucalypts produces high-quality monofloral honey.