In the game of rugby union, there are 15 players on each team, comprising eight forwards (numbered 1–8) and seven backs (numbered 9–15). In addition, there may be up to eight replacement players "on the bench". Jersey numbers 16–23 differentiate them. Players are not restricted to any single position on the field, although they generally specialise in just one or two that suit their skills and body types. Players that specialise in over three positions are called "utility players". The scrum (an assemblage used to restart play), however, must consist of eight players: the "front row" (two props, a loosehead and tighthead, and a hooker), the "second row" (two locks), and a "back row" (two flankers, and a number 8). The players outside the scrum are called "the backs": scrum-half, fly-half, two centres (inside and outside), two wings, and a fullback. Early names, such as "three-quarters" (for the wings and centres) and "outside-halves" (for fly-half) are still used by many in the Northern Hemisphere, while in the Southern Hemisphere the fly-half and inside centre are colloquially called "first five-eighth" and "second five-eighth" respectively, while the scrum-half is known as the "half-back".

Rugby union, or simply rugby, is a contact team sport which originated in England in the first half of the 19th century. One of the two codes of rugby football, it is based on running with the ball in hand. In its most common form, a game is between two teams of 15 players using an oval-shaped ball on a rectangular field with H-shaped goalposts on each try line.

In 1845, the first football laws were written by Rugby School pupils; other significant events in the early development of rugby include the Blackheath Club's decision to leave the Football Association in 1863 and the split between rugby union and rugby league in 1895. Historically an amateur sport, in 1995 restrictions on payments to players were removed, making the game openly professional at the highest level for the first time.

World Rugby, originally the International Rugby Football Board (IRFB) and from 1998 to 2014 the International Rugby Board (IRB), has been the governing body for rugby union since 1886. Rugby union spread from the Home Nations of Great Britain and Ireland, and was absorbed by many of the countries associated with the British Empire. Early exponents of the sport included Australia, New Zealand, South Africa and France. Countries that have adopted rugby union as their de facto national sport include Fiji, Georgia, Madagascar, New Zealand, Samoa, Tonga and Wales. Rugby union is played in over 100 countries across six continents; there are 101 full members and 18 associate members of World Rugby.