The Opiliones (formerly Phalangida) are an order of arachnids colloquially known as harvestmen, harvesters, harvest spiders, or daddy longlegs. As of April 2017, over 6,650 species of harvestmen have been discovered worldwide, although the total number of extant species may exceed 10,000. The order Opiliones includes five suborders: Cyphophthalmi, Eupnoi, Dyspnoi, Laniatores, and Tetrophthalmi, which were named in 2014.

Representatives of each extant suborder can be found on all continents except Antarctica.

Well-preserved fossils have been found in the 400-million-year-old Rhynie cherts of Scotland, and 305-million-year-old rocks in France. These fossils look surprisingly modern, indicating that their basic body shape developed very early on, and, at least in some taxa, has changed little since that time.

Their phylogenetic position within the Arachnida is disputed; their closest relatives may be the mites (Acari) or the Novogenuata (the Scorpiones, Pseudoscorpiones, and Solifugae). Although superficially similar to and often misidentified as spiders (order Araneae), the Opiliones are a distinct order that is not closely related to spiders. They can be easily distinguished from long-legged spiders by their fused body regions and single pair of eyes in the middle of the cephalothorax. Spiders have a distinct abdomen that is separated from the cephalothorax by a constriction, and they have three to four pairs of eyes, usually around the margins of the cephalothorax.

English speakers may colloquially refer to species of Opiliones as "daddy longlegs" or "granddaddy longlegs", but this name is also used for two other distantly related groups of arthropods, the crane flies of the family Tipulidae, and the cellar spiders of the family Pholcidae, (commonly referred to as "daddy long-leg spiders") most likely because of their similar appearance. Harvestmen are also referred to as "shepherd spiders" in reference to how their unusually long legs reminded observers of the ways that some European shepherds used stilts to better observe their wandering flocks from a distance.