Instant replay was first implemented in 1986, with the league voting to have another official in a booth with monitors to look over controversial decisions. The first ever use of this system was in the third play from scrimmage between the Chicago Bears and Cleveland Browns in which officials were unsure as to whether a poorly snapped ball had been properly recovered in the end zone. They went to the booth review and the touchdown was confirmed.
The replay system was not kind to everyone, as it overturned crucial plays and turned elation into misery as previously game-winning plays became meaningless. Players and teams began to oppose the system due to it slowing down the game and changing outcomes that by 1992 it was eradicated from the league.
It was not long before many irreversible missed calls occured that the NFL realized they needed the ability to review calls.
Under the new replay rules beginning in 1999, the coaches were in charge of when to look at a play, each having the ability to challenge if they disagreed with the call, rather than the officials being the ones to ask the review booth. Coaches are allotted two challenges with an extra possible if both are successful initially. An incorrect challenge leads to a timeout being stripped from the challenging team.
During a scoring play or turnover, or any play within two minutes of the end of a half, a challenge flag does not have to be thrown by a coach, rather each of these plays is automatically reviewed if there is some discrepancy.
Despite any complaints, instant replay is undoubtedly better for the NFL as it allows the right call to be made in more scenarios than just the official's on-field judgement.