<P> There are restrictions on what types of plays can be reviewed . In general, most penalty calls or lack thereof cannot be reviewed, nor can a play that is whistled dead by the officials before the play could come to its rightful end . </P> <P> American and Canadian football leagues vary in their application and use of instant replay review . </P> <P> The National Football League first adopted a limited Instant Replay system in 1986, though the current system began in 1999, bringing in the opportunity to "challenge" on - field calls of plays . The current system mirrors a system used by the now defunct USFL in 1985 . Each coach is allowed two opportunities per game to make a coach's challenge . Before the 2004 NFL season, the instant replay rule was slightly changed to allow a third challenge if both of the original two challenges were successful . </P> <P> A challenge can only be made on certain reviewable calls on plays that begin before the two - minute warning and only when a team has at least one time - out remaining in the half . Up until the 2005 season, coaches could signal a challenge via an electronic pager, now used only by the replay assistant . The coaches now throw a red flag onto the field, indicating the challenge to the referees . This red flag was originally the "backup plan" if the pager were to fail, but it has become the more popular option among NFL coaches to signal a replay request . </P>

How many challenges does a coach get in the nfl