The genus is widespread throughout the temperate and subtropical regions of the world. It includes species of trees, shrubs, and climbers, with evergreen or deciduous foliage and inconspicuous flowers. The genus was more extended in the tertiary and many species have adapted to laurel forest habitat. Ilex is adapted from sea level to more than 2,000 metres (6,600 ft) with high mountain species. It is dioecious with smooth, glabrous, or pubescent branchlets. The plants are generally slow-growing but over time can reach more than 10 m (33 ft). Many are evergreen trees with some species growing to 25 m (82 ft) tall. The type species is the European Ilex aquifolium described by Linnaeus,
Plants in this genus have simple, alternate glossy leaves, typically with a spiny toothed, or serrated leaf margin. The inconspicuous flower is greenish white, with four petals. Male and female commonly flower on different plants, although there are exceptions.