If a goalkeeper is injured or sent off, a substitute goalkeeper has to take their place, otherwise an outfield player must take the ejected keeper's place in goal. In order to replace a goalkeeper who is sent off, a team usually substitutes an outfield player for the backup keeper (thus effectively the red card and substitution takes out two of the starting eleven players). They then play the remainder of the match with nine outfield players. If a team does not have a substitute goalkeeper, or they have already used all of their permitted substitutions for the match, an outfield player has to take the dismissed goalkeeper's place and wear the goalkeeper shirt.
Goalkeepers often have longer playing careers than outfield players, many not retiring until their late thirties or early forties. This can be explained by noting that goalkeepers play a less physically demanding position that requires significantly less running. For example, Peter Shilton played for 31 years between 1966 and 1997 before retiring at the age of 47. Another explanation of this is that due to the nature of this position, goalkeepers use their experience more often than field players.
The squad number for a first choice goalkeeper is generally number 1, as the goalkeeper is the first player in a line-up, and is also the only position on the field that is required to be occupied.
Association football, like many sports, has experienced many changes in tactics resulting in the generation and elimination of different positions. Goalkeeper is the only position that is certain to have existed since the codification of the sport. Even in the early days of organised football, when systems were limited or non-existent and the main idea was for all players to attack and defend, teams had a designated member to play as the goalkeeper.
The earliest account of football teams with player positions comes from Richard Mulcaster in 1581 and does not specify goalkeepers. The earliest specific reference to keeping goal comes from Cornish Hurling in 1602. According to Carew: "they pitch two bushes in the ground, some eight or ten foot asunder; and directly against them, ten or twelve score off, other twayne in like distance, which they term their Goals. One of these is appointed by lots, to the one side, and the other to his adverse party. There is assigned for their guard, a couple of their best stopping Hurlers". Other references to scoring goals begin in English literature in the early 16th century; for example, in John Day's play The Blind Beggar of Bethnal Green (performed circa 1600; published 1659): "I'll play a gole at camp-ball" (an extremely violent variety of football, popular in East Anglia). Similarly, in a 1613 poem, Michael Drayton refers to "when the Ball to throw, And drive it to the Gole, in squadrons forth they goe". It seems inevitable that wherever a game has evolved goals, some form of goalkeeping must also be developed. David Wedderburn refers to what has been translated from Latin as to "keep goal" in 1633, though this does not necessarily imply a fixed goalkeeper position.