The Taconic Mountains (/təˈkɒnɪk/) are a 150-mile-long sub-range of the Appalachian Mountains lying on the eastern border of New York State and adjacent New England. The range, which played a role in the history of geological science, is separated from the Berkshires and Green Mountains to the east by a series of valleys, principally those of the Housatonic River, Battenkill River and Otter Creek. The Taconics' highest point is Mount Equinox in Vermont at 3,840 feet (1,170 m); among many other summits are Dorset Mountain, Mount Greylock and Mount Everett.

Forests are predominately maple-beech-birch with some spruce-fir at higher elevations, "and oak and hickory common to the south and at lower elevations." Parts of the Taconics are in the New England-Acadian forests ecoregion. Although mostly private property, the Taconics contain a half-dozen sizable state forests and parks, as well as many preserves of lesser acreage protected by land trusts. Several hundred miles of trails are within these mountains, including parts of the Appalachian Trail.

Taconic is likely from a Delaware word meaning "in the trees". The spelling "Taghkanic", among several variant transliterations, is current for about a half-dozen locales, mostly within the immediate region. As written English, the term appears in a 1685 petition for the right to purchase land in western Massachusetts. Timothy Dwight IV used the name "Taughkannuc Mountain" in an account of his 1781 ascent of the summit later named Mount Everett. Dwight's account was deemed the "first recorded ascent" of that peak in a 1989 text on regional hiking history.

The term first entered geological literature in 1819 with Chester Dewey's "Sketch of the Mineralogy and Geology of the Vicinity of Williams' College". It gained prominence in the field when Dewey's protégé Emmons proposed the existence of a "Taconic System" in 1839 (see above). In the 20th century, it became attached to the theory of an Ordovician mountain-building event involving much of what is now eastern North America named the "Taconic Orogeny".

The range is part of the Taconic Allochthon, a local rock structure which traveled to its current position from about 25 miles to the east through low-angle thrusting. Rocks of this allochthon are older than the strata lying beneath, and consist of slate, phyllite, and schist, "plus some minor lenses of limestone." The Taconic allochthon is larger than the current Taconic Mountains, extending westward toward the Hudson River. It formed during the Ordovician period in the collision of the North American Plate into a volcanic island arc.

Geologist E-An Zen proved this allochthon's existence in 1966 via a study of the region's faulting, although it had been proposed around 1906 by Rudolf Ruedemann and separately by Arthur Keith, each using concepts associated with French geologist Marcel Alexandre Bertrand. Their proposals regarding the Taconics, however, were viewed as unproven and controversial until Zen's work.

An earlier and largely separate dispute over the age of the Taconic Mountains based on the region's sparse fossil record began in the 1830s and centered on the theory of Ebenezer Emmons of Williams College that the range was older than its surroundings with a distinct geology he named "the Taconic System." In opposing Emmons' theory, James Hall of Rensselaer Polytechnic Institute initially gained prominent support from famed international scientists of the day, including Louis Agassiz and Charles Lyell.

Emmons died in 1863, yet as late as the 1880s two leading geologists of the era James Dwight Dana (Yale University) and Jules Marcou wrote of their continued and sometimes bitter disagreements over Emmons' theory. This early phase of ideas about the Taconic Mountains is compared with the Great Devonian Controversy that preoccupied British geologists for part of the 19th century.