Lemurs ( (listen) LEE-mər) (from Latin lemures – ghosts or spirits) are wet-nosed primates of the superfamily Lemuroidea (), divided into 8 families and consisting of 15 genera and around 100 existing species. They are endemic to the island of Madagascar. Most existing lemurs are small, have a pointed snout, large eyes, and a long tail. They chiefly live in trees and are active at night.

Lemurs share resemblance with other primates, but evolved independently from monkeys and apes. Due to Madagascar's highly seasonal climate, lemur evolution has produced a level of species diversity rivaling that of any other primate group. Until shortly after humans arrived on the island around 2,000 years ago, there were lemurs as large as a male gorilla. Most species have been discovered or promoted to full species status since the 1990s; however, lemur taxonomic classification is controversial and depends on which species concept is used.

Lemurs range in weight from the 30-gram (1.1 oz) mouse lemur to the 9-kilogram (20 lb) indri. Lemurs share many common basal primate traits, such as divergent digits on their hands and feet, and nails instead of claws (in most species). However, their brain-to-body size ratio is smaller than that of anthropoid primates. As with all strepsirrhine primates, they have a "wet nose" (rhinarium). Lemurs are generally the most social of the strepsirrhine primates, and communicate more with scents and vocalizations than with visual signals. Lemurs have a relatively low basal metabolic rate, and as a result may exhibit dormancy such as hibernation or torpor. They also have seasonal breeding and female social dominance. Most eat a wide variety of fruits and leaves, while some are specialists. Two species of lemurs may coexist in the same forest due to different diets.

Lemur research during the 18th and 19th centuries focused on taxonomy and specimen collection. Modern studies of lemur ecology and behavior did not begin in earnest until the 1950s and 1960s. Initially hindered by political issues on Madagascar during the mid-1970s, field studies resumed in the 1980s. Lemurs are important for research because their mix of ancestral characteristics and traits shared with anthropoid primates can yield insights on primate and human evolution. Many lemur species remain endangered due to habitat loss and hunting. Many lemur species have already gone extinct in the last 2000 years due to human activity, and are collectively referred to as the "subfossil lemurs". These are typically larger than extant lemurs, with the largest, Archaeoindris, being the size of a gorilla. Although local traditions, such as fady, generally help protect lemurs and their forests, illegal logging, economic privation and political instability conspire to thwart conservation efforts. Because of these threats and their declining numbers, the International Union for Conservation of Nature (IUCN) considers lemurs to be the world's most endangered mammals, noting that as of 2013 up to 90% of all lemur species confront the threat of extinction in the wild within the next 20 to 25 years. As an iconic flagship species that exemplifies the biodiverse fauna of Madagascar, however, lemurs have facilitated the emergence of eco-tourism in Madagascar in World Heritage Sites, such as the Rainforests of the Atsinanana in eastern Madagascar. In addition, conservation organizations, such as the Lemur Conservation Foundation and the Duke Lemur Center, increasingly seek to implement community-based approaches, such as encouraging local communities to adopt sustainable agriculture and afforestation initiatives, to expand employment opportunities for ecological programs, preserve lemur habitats as well as promote public awareness and appreciation for lemurs.