Fork-marked lemurs or fork-crowned lemurs are strepsirrhine primates; the four species comprise the genus Phaner. Like all lemurs, they are native to Madagascar, where they are found only in the west, north, and east sides of the island. They are named for the two black stripes which run up from the eyes, converge on the top of the head, and run down the back as a single black stripe. They were originally placed in the genus Lemur in 1839, later moved between the genera Cheirogaleus and Microcebus, and given their own genus in 1870 by John Edward Gray. Only one species (Phaner furcifer) was recognized, until three subspecies described in 1991 were promoted to species status in 2001. New species may yet be identified, particularly in northeast Madagascar.

Fork-marked lemurs are among the least studied of all lemurs and are some of the largest members of the family Cheirogaleidae, weighing around 350 grams (12 oz) or more. They are the most phylogenetically distinct of the cheirogaleids, and considered a sister group to the rest of the family. Aside from their dorsal forked stripe, they have dark rings around their eyes, and large membranous ears. Males have a scent gland on their throat, but only use it during social grooming, not for marking territory. Instead, they are very vocal, making repeated calls at the beginning and end of the night. Like the other members of their family, they are nocturnal, and sleep in tree holes and nests during the day. Monogamous pairing is typical for fork-marked lemurs, and females are dominant. Females are thought to have only one offspring every two years or more.

These species live in a wide variety of habitats, ranging from dry deciduous forests to rainforests, and run quadrupedally across branches. Their diet consists primarily of tree gum and other exudates, though they may obtain some of their protein and nitrogen by hunting small arthropods later at night. All four species are endangered. Their populations are in decline due to habitat destruction. Like all lemurs, they are protected against commercial trade under CITES Appendix I.

Fork-marked lemurs were first documented in 1839 by Henri Marie Ducrotay de Blainville when he described the Masoala fork-marked lemur (P. furcifer) as Lemur furcifer. The holotype is thought to be MNHN 1834-136, a female specimen taken from Madagascar by French naturalist Jules Goudot. The source of this specimen is unknown, but thought to be Antongil Bay. In 1850, Isidore Geoffroy Saint-Hilaire moved the fork-marked lemurs to the genus Cheirogaleus (dwarf lemurs), but they were also commonly listed in the genus Microcebus (mouse lemurs). In 1870, John Edward Gray assigned fork-marked lemurs to their own genus, Phaner, after initially including them and the mouse lemurs in the genus Lepilemur (sportive lemurs). Although French naturalist Alfred Grandidier accepted Gray's new genus (while also lumping the other cheirogaleids in Cheirogaleus and illustrating the cranial similarities between cheirogaleids and Lepilemur) in 1897, the genus Phaner was not widely accepted. In the early 1930s, Ernst Schwarz, Guillaume Grandidier, and others resurrected the name, citing characteristics that were intermediate between Cheirogaleus and Microcebus.

Until the late 20th century, there was only one recognized species of fork-marked lemur, although size and coloration differences had been noted previously. After comparing museum specimens, paleoanthropologist Ian Tattersall and physical anthropologist Colin Groves recognized three new subspecies in 1991: the Pale fork-marked lemur (P. f. pallescens), Pariente's fork-marked lemur (P. f. parienti), and the Amber Mountain fork-marked lemur (P. f. electromontis). In 2001, Groves elevated all four subspecies to species status based on noticeable color, size, and body proportion differences between the fragmented populations. Although Tattersall disagreed with this promotion, citing inadequate information for the decision, the arrangement is generally accepted.

In December 2010, Russell Mittermeier of Conservation International and conservation geneticist Edward E. Louis Jr. announced the possibility of a new species of fork-marked lemur in the protected area of Daraina in northeast Madagascar. In October, a specimen was observed, captured, and released, although genetic tests have yet to determine if it is a new species. The specimen demonstrated a slightly different color pattern from other fork-marked lemur species. If shown to be a new species, they plan to name it after Fanamby, a key conservation organization working in that protected forest.

The etymology of the genus Phaner puzzled researchers for many years. Gray often created mysterious and unexplained taxonomic names. In 1904, Theodore Sherman Palmer attempted to document the etymologies of all mammalian taxa, but could not definitively explain the origins of the generic name Phaner, noting only that it derived from the Greek φανερός (phaneros) meaning "visible, evident". In 2012, Alex Dunkel, Jelle Zijlstra, and Groves attempted to solve the mystery. Following some initial speculation, a search of the general literature published around 1870 revealed the source: the British comedy The Palace of Truth by W. S. Gilbert, which premiered in London on 19 November 1870, nearly one and a half weeks prior to the date written on the preface of Gray's manuscript (also published in London). The comedy featured characters bearing three names: King Phanor (sic), Mirza, and Azema. Since the genera Mirza (giant mouse lemurs) and Azema (for M. rufus, now a synonym for Microcebus) were both described in the same publication and equally enigmatic, the authors concluded that Gray had seen the comedy and then based the names of three lemur genera on its characters.

Fork-marked lemurs were called "fork-marked dwarf lemurs" by Henry Ogg Forbes in 1894 and "fork-crowned mouse lemur" by English missionary and naturalist James Sibree in 1895. Literature searches by Dunkel et al. also uncovered other names, such as "fork-lined lemur" and "squirrel lemur", during the early 1900s. By the 1970s, reference to dwarf and mouse lemurs had ended, and the "fork-crowned" prefix became popular between 1960 and 2001. Since then, the "fork-marked" prefix has become more widely used. These lemurs get their common name from the distinctive forked stripe on their head.