It is a relatively large and robust sandpiper and is the largest of the species called "shanks" in the genus Tringa.

Its closest relative is the lesser yellowlegs, a much smaller bird with a very different appearance apart from the fine, clear, and dense pattern of the neck, which both species show in breeding plumage.

The willet was formally described in 1789 by the German naturalist Johann Friedrich Gmelin in his revised and expanded edition of Carl Linnaeus's Systema Naturae.

[2] Gmelin based his description on the "semipalmated snipe" from New York that had been described in 1785 by both the English ornithologist John Latham and by the Welsh naturalist Thomas Pennant.

[5][6] In 2006 it was moved to the current genus Tringa based on a molecular phylogenetic study published the previous year.

[7][8] The genus name Tringa is the Neo-Latin word given to the green sandpiper by the Italian naturalist Ulisse Aldrovandi in 1603 based on Ancient Greek trungas, a thrush-sized, white-rumped, tail-bobbing wading bird mentioned by Aristotle.

[13] The willet is a very rare vagrant to Europe with records from the Azores, mainland Portugal, France, Norway and Finland.

The male and female may form a long-term monogamous bond and return to the same territory in successive breeding seasons.

The western subspecies is threatened by the conversion of native grasslands and drainage of wetlands for agriculture while the wintering habitat has been degraded locally in California by coastal development.

Willets are also vulnerable to being killed by colliding with power lines laid through their wetland breeding areas.

The passage of the Migratory Bird Treaty Act in 1918 protected the willet from intensive exploitation by hunters for food and allowed its numbers to rebound to present levels.