Achillea millefolium is an erect, herbaceous, perennial plant that produces one to several stems 0.2–1 metre (8–40 inches) in height, and has a spreading rhizomatous growth form.

Cauline and more or less clasping,[2] the leaves appear spirally and evenly along the stem, with the largest and most petiolate towards the base;[3] they are 5–20 centimetres (2–8 in) long and fernlike, divided bipinnately or tripinnately.

[4] The inflorescence has 4 to 9 phyllaries and contains ray and disk flowers which are white to pink, blooming from March to October.

[10] The several varieties and subspecies include: The genus name Achillea is derived from mythical Greek character Achilles, who reportedly carried it with his army to treat battle wounds.

[20][21] Common yarrow produces an average yield of 110,000 plants per hectare (43,000/acre), with a total dry weight of 11,800 kg/ha (10,500 pounds per acre).

[22] It has been introduced as a feed for livestock in New Zealand[23] and Australia, where it is a common weed of both wet and dry areas, such as roadsides, meadows, fields and coastal places.

[25] The larvae of the moths Bucculatrix clavenae, B. cristatella, B. fatigatella, B. humiliella, B. latviaella, Cnephasia abrasana, Cochylimorpha elongana, Coleophora argentula, C. carelica, C. ditella, C. expressella, C. follicularis, C. gardesanella, C. millefolii, C. partitella, C. ptarmicia, C. quadristraminella, C. succursella, C. vibicigerella, Depressaria olerella, D. silesiaca, Dichrorampha alpinana (broad-blotch drill), D. petiverella, D. vancouverana (tanacetum root moth), Eupithecia millefoliata (yarrow pug), E. nanata (narrow-winged pug), Gillmeria pallidactyla, Idaea pallidata, Isidiella nickerlii, Loxostege manualis, Phycitodes maritima, P. saxicola, Pyncostola bohemiella, Sophronia sicariellus and Thetidia smaragdaria (Essex emerald) feed on Achillea millefolium in Europe.

The larvae of Chlorochlamys chloroleucaria (blackberry looper), Coleophora quadruplex and Sparganothoides lentiginosana (lentiginos moth) feed on A. millefolium in North America.

Other species of moths with a more cosmopolitan distribution include Aethes smeathmanniana (Smeathmann's aethes moth), Chloroclystis v-ata (v-pug), Choristoneura diversana, Cochylidia richteriana, Epiblema graphana, Eupithecia succenturiata (bordered pug), E. vulgata (common pug), Jordanita budensis and Thiodia citrana (lemon bell).

[26] Cassida denticollis, Galeruca tanaceti, Hypocassida subferruginea and Phytoecia virgula are cosmopolitan species of beetles that feed on A. millefolium.

Hedychrum rutilans is a species of cuckoo wasps whose adults can be found feeding on A. millefolium in Europe and North Africa.

[49] According to the American Society for the Prevention of Cruelty to Animals, yarrow is toxic to dogs, cats, and horses, causing increased urination, vomiting, diarrhea and dermatitis.

[51] In a standard rodent model for reproductive toxicity, aqueous extracts of yarrow produced a significant increase in the percentage of abnormal sperm.

[56] Native American nations used the plant for healing cuts and abrasions, relief from earaches and throat infections, as well as for an eyewash.

[61] In the Middle Ages, yarrow was part of a herbal mixture known as gruit used in the flavoring of beer prior to the use of hops.

[64] Yarrow has been found with Neanderthal burials, suggesting its association with human species dates to at least 60,000 years ago.

[67] In the Classical Greek epic Iliad, Homer tells of the centaur Chiron, who conveyed herbal secrets to his human pupils and taught Achilles to use yarrow on the battlegrounds of Troy.

[55] Other names implying the plant's historical use in healing—particularly in the military—include bloodwort, knight's milfoil, staunchweed, and, from its use in the United States Civil War, soldier's woundwort.

[53] It was called old man's pepper due to its pungent flavor, while the name field hop came from its use in beer making in Sweden.

[71] In the witchcraft trial of Elspeth Reoch in March 1616, she was alleged to have plucked "melefour", thought to be another name for yarrow, and said "In nomine Patris, Fiili, et Spiritus Sancti" to become able to cure distemper (disorders of the four humours) and impart the faculty of prediction.

[53] Yarrow was thought to bring luck due to being, according to one woman cited by James Britten (c. 1878), "the first herb our Saviour put in His hand when a child".