助理裁判员是指在足球比赛中辅助主裁判控制比赛的人员，其中两个站在边线外的助理裁判员一般也称为边裁（或称为旁证），其主要职责是当球整体离开场地时，示意由哪一方踢角球、球门球或掷界外球；判罚处于越位位置的队员；以及示意发生主裁判视线外的不正当行为或任何其他事件；踢点球时，示意在球被踢出之前守门员是否向前移动，以及球被踢出后是否进门。助理裁判员还包括负责比赛中换人和赛场管理的第四官员（或称为第四球证），如果主裁判和两名边裁中的任何一位不能继续执行执法工作时，第四官员可以上场替补。

In association football, an assistant referee (known as a linesman or lineswoman before 1996) is an official empowered with assisting the referee in enforcing the Laws of the Game during a match. At most organised levels of football the match officiating crew consists of the referee and two assistant referees, with one assistant referee assigned to each touchline. The assistant referee's duties generally consist of judging when the ball has left the field of play – including which team is entitled to return the ball to play, judging when an offside offence has occurred, and advising the referee when an infringement of the Laws of the Game has occurred out of his or her view. Depending on the local match rules and the discretion of the referee, an assistant referee may also be responsible for various administrative tasks, for example managing substitutions, helping the referee control the players, or replacing the referee if he or she is unable to continue. The assistant referee functions in an advisory role, and all judgements made by an assistant referee may be overruled by the referee.