In the game of rugby union, there are 15 players on each team, comprising eight forwards (wearing jerseys numbered 1–8) and seven backs (numbered 9–15). In addition, there may be up to eight replacement players "on the bench", numbered 16–23. Players are not restricted to a single position, although they generally specialise in just one or two that suit their skills and body types. Players that play multiple positions are called "utility players".

The scrum (a contest used to restart play) must consist of eight players from each team: 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, inside centre, outside centre, two wings, and a fullback.

Forwards compete for the ball in scrums and line-outs and are generally bigger and stronger than the backs. Props push in the scrums, while the hooker tries to secure the ball for their team by "hooking" it back with their foot. The hooker is also usually responsible for throwing the ball in at line-outs, where it is mostly competed for by the locks, who are generally the tallest players on the team. The flankers and number eight are expected to be the first players to arrive at a breakdown and play an important role in securing possession of the ball for their team.

The backs play behind the forwards and are usually more lightly built and faster. Successful backs are skilful at passing and kicking. Full-backs need to be good defenders and kickers, and have the ability to catch a kicked ball. The wingers are usually among the fastest players in a team, and score many of the tries. The centres' key attacking roles are to break through the defensive line and link successfully with wingers. The fly-half can be a good kicker and generally directs the back line. The scrum-half retrieves the ball from the forwards and needs a quick and accurate pass to get the ball to the backs (often first to the fly-half).

Early names, such as "three-quarters" (for the wings and centres) and "outside-half" or simply "out-half" (for fly-half) are sometimes used in the Northern Hemisphere, while in New Zealand the fly-half and inside centre are called "first five-eighth" and "second five-eighth" respectively, while the scrum-half is known as the "half-back".

There is a maximum of 15 players from each team allowed on a rugby field at any one time. The players' positions at the start of the game are indicated by the numbers on the backs of their shirts, 1 to 15. The positions are divided into two main categories; forwards (numbered 1 to 8) and backs (numbered 9 to 15). In international matches, there are eight substitutes that can replace an on-field team-mate. The substitutes, numbered 16 to 23, can either take up the position of the player they replace or the on-field players can be shuffled to make room for this player in another position. Typically, the forwards among the substitutes will have lower numbers than the backs. There are no personal squad numbers and a versatile player's position and number may change from one game to the next. Players can also change positions during the match; common examples are the fly-half playing the full-back's position in defence or a prop taking the hooker's position at line-outs.

Different positions on the field suit certain skill sets and body types, generally leading to players specialising in a limited number of positions. Each position has certain roles to play on the field, although most have been established through convention rather than law. During general play, as long as they are not offside, the players may be positioned anywhere on the field. It is during the set pieces (scrum and line-out) that the positions are enforced.

During early rugby union games there were only really two positions; most players were in the forwards, who formed part of the scrimmage (which later was called "scrummage" and then "scrum"), and a few defensive "tends" (from "goaltenders").