Being a backup quarterback can be one of the most demanding jobs in the world of sports. Backups are expected to know the entire playbook, develop a rapport with their receivers, and be a leader on and off the field, while having far fewer repetitions in practice. Holding a clipboard on Sunday afternoons may seem like a good way to make a living, but there is always the possibility that a backup must take over when circumstances deem it necessary.

A qualified backup quarterback is a luxury for NFL teams and many are making significant investments in their backups. Already this NFL season, we have seen several backup quarterbacks turn in incredible performances. Mark Sanchez is the latest backup to emerge as a competent starter and his performance is already causing pundits to re-evaluate his status. Several legendary teams would not hold that distinction if it were not for the performance of their quarterbacks in waiting.

A backup carries a lot of the same pressure that starting quarterbacks feel. Backups are often viewed as a potential upgrade over their current starter. They can be called upon in an attempt to bring their teams back into contention in dire circumstances. When these quarterbacks step onto the field they will face the same defense as the starter, but with far less time to prepare. These men have risen to the occasion to accomplish some of the most amazing feats ever seen on a football field.

15 15. Randall Cunningham – Minnesota Vikings

Randall Cunningham spent nearly a decade under center as the starting quarterback for the Philadelphia Eagles. Following a brief retirement during the 1996 season, Cunningham returned as a backup quarterback for the Minnesota Vikings. In 1998, when Brad Johnson went down with a broken leg in Week 2, Cunningham stepped in and had the best season of his NFL career. He led the Vikings to a 15-1 regular season record, while chucking touchdown passes to Cris Carter and Randy Moss (who was a rookie). The Vikings eventually lost to the Atlanta Falcons in the NFC Championship game and traded away Brad Johnson in the offseason.

14 14. Tony Romo – Dallas Cowboys

After going undrafted and spending two seasons as a holder for the Dallas Cowboys, Tony Romo was finally given an opportunity to start in his third NFL season. Romo has been the Cowboys starter ever since and broken several team records along the way. He has earned three Pro Bowl appearance, finished second in the NFL in passing touchdowns in 2007 and third in passing yardage in 2009. Despite his reputation of not being a clutch player, Romo has the highest 4th quarter passer rating of any quarterback in the league since 2006.

13 13. Jeff Hostetler – New York Giants

The New York Giants under Bill Parcells were a juggernaut. Jeff Hostetler spent the better part of five seasons as the backup to Phil Simms, before he was finally given a chance to shine in the 1990 season. Hostetler had been frustrated as a backup and considered retirement, but after Simms suffered a foot injury, he was given the starting job for the end of the campaign. Hostetler helped the Giants to five consecutive wins as a starter, including a win over the Buffalo Bills in Super Bowl XXV. Following the Super Bowl victory, Hostetler decided not to retire and spent seven more seasons in the NFL. He was mediocre the rest of his career, but he's a legend in New York after his Super Bowl win.

12 12. Trent Dilfer – Baltimore Ravens

Trent Dilfer came to the Baltimore Ravens as the backup for Tony Banks, after leading the Tampa Bay Buccaneers for six years. Dilfer entered a Week 8 game for Banks who had thrown three interceptions against the Tennessee Titans. Dilfer was unable to help the Ravens win in Week 8, and they struggled in Week 9. However, Dilfer then rattled off eleven consecutive wins as the Ravens starter and helped them win the Super Bowl. Dilfer threw for over 250 yards only once during that 13 game stretch and holds the honor of being considered one of the best game manager quarterbacks of all time.

11 11. Doug Williams – Washington Professional Football Team

Doug Williams entered the NFL as a highly touted prospect from Grambling, and the Tampa Bay Buccaneers drafted him with the 17th overall pick. Williams spent five seasons as the Bucs starter before jumping ship for the USFL. When the USFL folded, he was persuaded to join Washington by head coach Joe Gibbs. He began as a backup to Jay Schroeder and didn’t start a game in his first season. During the 1987 season, Schreoder was injured and Willaims stepped in as the starter. That season, Williams became the first black quarterback in NFL history to win the Super Bowl. He returned as the starter in 1988, but wasn't great and was out of the league by 1990. He's still a legend in Washington though.

10 10. Brett Favre – Green Bay Packers

After being drafted by the Atlanta Falcons in the second round of the 1991 NFL Draft, Falcons head coach Jerry Glanville said it would take a plane crash for Favre to start. Favre was traded the following season to the Green Bay Packers, despite doctors recommending the Packers reject the trade due to a failed physical. Favre went on to become one of the most prolific passers in the history of the NFL and earned 11 Pro Bowl appearances. He won three consecutive MVP Awards from 1995 to 1997 and won Super Bowl XXXI. If Glanville had his way, America never would have fallen in love with the Gunslinger.

9 9. Terry Bradshaw – Pittsburgh Steelers

Terry Bradshaw is now known as the quarterback who led four Pittsburgh Steelers teams to Super Bowl victories, but he wasn’t always the man in Pittsburgh. Despite being the 1st overall pick in the 1970 NFL Draft, Bradshaw began his career as a backup to Terry Hanratty. It took Bradshaw some time to adjust to the speed and pressure of the NFL, but when he did, he delivered several of the most memorable performances in the history of the sport.

8 8. Doug Flutie – Buffalo Bills

After winning the Heisman Trophy in his senior season with Boston College, Doug Flutie was at the center of the war between the NFL and USFL. While the NFL’s Buffalo Bills had doubts about Flutie’s height, the USFL doubled down and made him the highest paid rookie in any sport. The league soon folded and Flutie had unsuccessful stints with the Bears and Patriots before becoming a legend in the Canadian Football League. Flutie returned to the NFL in 1998 and won the starting job after Rob Johnson went down with an injury. Flutie went 7-3 as a starter, earning a Pro Bowl appearance and NFL Comeback Player of the Year.

7 7. Kurt Warner – St. Louis Rams

Kurt Warner’s journey to the NFL began after he was cut from the Green Bay Packers during a pre-season try out. He then went from stocking boxes at a grocery store to throwing touchdown passes in the Arena Football League and NFL Europe, before he was given a chance as a backup on the St. Louis Rams. When starting quarterback Trent Green suffered a torn ACL in preseason, Dick Vermeil named Warner as the starter. Warner led the Rams to a 13-3 record in the 1999 season, at the head of the offense known as the Greatest Show on Turf. The Rams won the Super Bowl that season, and Warner was named Super Bowl MVP and NFL MVP.

6 6. Earl Morrall – Baltimore Colts, Miami Dolphins

Earl Morrall may be the ultimate backup quarterback. During his 21 seasons in the NFL, he won three Super Bowl titles with two different teams, playing a crucial role in two of those wins. In the 1968 season, Morrall stepped in for an injured Johnny Unitas and led his team to an appearance in Super Bowl III. Two years later, in Super Bowl V, he replaced an injured Unitas and actually won the big one. Two years after that, he stepped in for the injured Bob Griese and led the Miami Dolphins to the only perfect season in NFL history. Morrall was replaced by Griese after struggling against the Pittsburgh Steelers’ Steel Curtain defense in the AFC Championship Game. Morrall earned the inaugural Comeback Play of the Year Award for his efforts. He then won another Super Bowl as a backup to Griese in 1973.

5 5. George Blanda – Oakland Raiders

George Blanda played more NFL seasons than any player in history. His 26 seasons in the NFL were spent as a kicker and quarterback for several teams. He won three AFL Championships with the Houston Oilers and Oakland Raiders during the 1960s. During the 1970 season with the Oakland Raiders, he was utilized in a similar fashion to a relief pitcher for regular starting quarterback Daryle Lamonica.  Blanda stepped in for Lamonica during the 1970 AFC Championship Game, where he threw two touchdowns, kicked a 48-yard field goal, and two extra points while becoming the oldest quarterback to play a championship game.

4 4. Jim Plunkett – Oakland Raiders

After spending seven seasons as a routine starter for the New England Patriots and San Francisco 49ers, Jim Plunkett joined the Oakland Raiders as a backup in 1978. After limited playing time over the next two seasons, Plunkett was given the starting job when Dan Pastorini injured himself. Plunkett led the Raiders to a 9-2 record in the regular season before going 4-0 in the postseason and winning Super Bowl XV, earning the Super Bowl MVP award. Plunkett repeated the feat in 1983, when he replaced Marc Wilson and led the Raiders to a Super Bowl XVIII victory.

3 3. Aaron Rodgers – Green Bay Packers

Aaron Rodgers was expected to be the first overall pick in the 2005 NFL Draft, but slid down the board to the Green Bay Packers who selected him with the 24th overall pick. Rodgers spent three seasons as a backup before being given the starting job. Since earning the role, Rodgers has become the best quarterback in the NFL. He led the Packers to a Super Bowl XLV victory and was named MVP of the game. He followed it up with an NFL MVP season during the 2011 campaign. Rodgers is one of the most consistent quarterbacks in the game and has become a beloved figure among Packers fans.

2 2. Steve Young – San Francisco 49ers

Steve Young spent two very unsuccessful seasons with the Tampa Bay Buccaneers before he was traded to the San Francisco 49ers to serve as Joe Montana’s backup. Young had several fantastic performances while stepping in for the injured Montana and earned the starting job when Montana suffered a pre-season injury in 1991. The 49ers missed the post-season that year despite a 10-6 record. The following year, the 49ers traded Montana in order for Young to become the team’s starter. He rewarded their faith in the 1994 season, during which he led the team to a 13-3 record and a Super Bowl title.

1 1. Tom Brady – New England Patriots

Tom Brady entered the NFL with a chip on his shoulder after being selected 199th overall at the 2000 NFL Draft. Brady climbed the depth chart in his first season and replaced the injured starter Drew Bledsoe in Week 3 of the 2001 season. The Patriots went 11-3 with Brady as a starter, and he earned an invitation to the Pro Bowl for his efforts. The 2001 Playoffs is where Brady really proved his mettle with a string of heroic performances against the Raiders, Steelers, and in the Super Bowl against the heavily favored Rams. Brady and the Patriots were victorious in Super Bowl XXXVI, and he earned the MVP honors. He has since won two more Super Bowls and continues to be one of the league’s biggest stars.