They have four or five sepals, four or five petals about 1–6 mm long (absent in some species), four to ten stamens about 6–10 mm long, and two pistils or a pistil with two styles. The flowers are regular, pentamerous, and borne in racemes, corymbs, or umbels. Maple species, such as Acer rubrum, may be monoecious, dioecious or polygamodioecious. A few, such as Acer laevigatum (Nepal maple) and Acer carpinifolium (hornbeam maple), have pinnately veined simple leaves. One species, Acer negundo (box-elder or Manitoba maple), has pinnately compound leaves that may be simply trifoliate or may have five, seven, or rarely nine leaflets. Several species, including Acer griseum (paperbark maple), Acer mandshuricum (Manchurian maple), Acer maximowiczianum (Nikko maple) and Acer triflorum (three-flowered maple), have trifoliate leaves. A small number of species differ in having palmate compound, pinnate compound, pinnate veined or unlobed leaves. The leaves in most species are palmate veined and lobed, with 3 to 9 (rarely to 13) veins each leading to a lobe, one of which is central or apical. Maples are distinguished by opposite leaf arrangement. Acer circinatum (vine maple) leaves showing the palmate veining typical of most species A few species, notably Acer cappadocicum, frequently produce root sprouts, which can develop into clonal colonies. Many of the root systems are typically dense and fibrous, inhibiting the growth of other vegetation underneath them. There are a few exceptions such as sugar maple. Most are shade-tolerant when young and are often riparian, understory, or pioneer species rather than climax overstory trees. Most species are deciduous, and many are renowned for their autumn leaf colour, but a few in southern Asia and the Mediterranean region are evergreen. Others are shrubs less than 10 meters tall with a number of small trunks originating at ground level. Most maples are trees growing to a height of 10–45 m (33–148 ft). The oldest fossils of Acer in Europe are from Svalbard, dating to the late Eocene ( Priabonian ~38-34 million years old). The oldest known fossils of Acer are from the late Paleocene of Northeast Asia and northern North America, around 60 million years old. The closest relative of Acer is Dipteronia, which only has two living species in China, but has a fossil record extending back to the middle Paleocene in North America. Numerous maple species are widely grown in gardens where many are especially valued for their autumn colour. It is one of the most common genera of trees in Asia. Maple syrup is made from the sap of some maple species. The closest relatives of the maples are the horse chestnuts. Maples usually have easily recognizable palmate leaves ( Acer negundo is an exception) and distinctive winged fruits.

The type species of the genus is the sycamore maple, Acer pseudoplatanus, the most common maple species in Europe. Only one species, Acer laurinum, extends to the Southern Hemisphere. There are approximately 132 species, most of which are native to Asia, with a number also appearing in Europe, northern Africa, and North America. The genus is placed in the family Sapindaceae.

Acer pseudoplatanus (sycamore maple) foliageĪcer ( / ˈ eɪ s ər/) is a genus of trees and shrubs commonly known as maples.

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