As if earning an NFL head-coaching job isn't difficult enough, an even greater challenge is keeping that position.

Only 16 coaches in league history have reached 20 seasons as the man in charge. The next in line may include Pete Carroll, Mike Tomlin, Mike McCarthy and John Harbaugh, but only two active coaches fit the billing.

This is a tough business to crack.

If you follow the NFL, you likely recognize Bill Belichick and Andy Reid in the accompanying picture. They are two of just six coaches since the AFL-NFL merger in 1970 to be the boss for 23-plus years.

Four others who largely coached before the merger are included as honorable mentions.

George Halas (40 years): Few people in football history have a more interesting story than George Halas, who served as a player, coach and executive of the Chicago Bears. His coaching tenures ran from 1920-29 to 1933-42 and 1946-55 to 1958-67. Along the way, Halas won 316 games and guided the Bears to six league championships.

Curly Lambeau (33 years): The namesake of the Green Bay Packers' stadium, Curly Lambeau had a prolific career in the same era as Halas. Lambeau coached the Packers from 1921-49, compiling 209 victories and six NFL titles. He also spent two seasons at the helm of the Chicago Cardinals and Washington franchises in 1950-51 and 1952-53.

Paul Brown (25 years): Paul Brown is remembered as the catalyst for the integration of football. After leading the Cleveland Browns to AAFC championships in 1946-49, he oversaw NFL-winning squads in 1950, 1954 and 1955. Brown collected 158 wins in 17 seasons with Cleveland and added 55 more after co-founding the Cincinnati Bengals in 1968.

Steve Owen (24 years): Throughout his career from 1930-53, Steve Owen stayed with the New York Giants. He went 153-100-17 in 24 seasons, helping the franchise win championships in 1934 and 1938.

For more than three decades, Chuck Noll stood as the most accomplished coach in NFL history.

Noll engineered a brilliant six-year stretch with the Pittsburgh Steelers, a franchise he elevated from 1-13 in 1969 to a four-time champion in the 1970s. Pittsburgh won back-to-back Super Bowls in 1974-75 and 1978-79.

Ultimately, he spent 23 seasons in charge of the Steelers. They registered a 193-148-1 record in Noll's tenure and made the playoffs 12 times with nine division titles.

Noll entered the Pro Football Hall of Fame in 1993.

Following an eight-year playing career with the Dallas Cowboys, Dan Reeves swapped out his helmet for a headset.

Reeves paid his dues as an assistant in Dallas, landing his first head-coaching spot with the Denver Broncos in 1981. He guided the Broncos—who added Hall of Fame quarterback John Elway in the iconic 1983 draft—for 12 seasons with substantial success. Denver notched a 110-73-1 record and three AFC championships, though it didn't win a Super Bowl.

Denver moved on from Reeves in 1993, when he landed with the Giants for a disappointing 31-33 four-year run.

Reeves finished his coaching career with the Atlanta Falcons. Again, it didn't go particularly well (49-59-1 in seven seasons), but the Falcons at least won the NFC in 1998.

Altogether, he posted a 190-165-2 record in 23 seasons.

In short, Andy Reid has just won.

"Big Red" took control of the Philadelphia Eagles in 1999 and quickly turned the franchise into an annual power. Philadelphia won 11-plus games from 2000-04, reaching the NFC Championship Game in the last four seasons and finally reaching the Super Bowl in 2004.

However, the Eagles could not manage that final victory, which led to his departure after the 2012 season. Reid totaled 130 victories and six division crowns in 14 years with Philly.

He's always won. But with the Kansas City Chiefs, he's won big.

That, of course, is a large credit to Patrick Mahomes. Nevertheless, the Chiefs have tallied nine-plus victories in all 10 seasons of Reid's tenure—and five happened before Mahomes as their quarterback.

Reid and Mahomes have propelled Kansas City to five straight AFC West crowns and AFC Championship Game appearances, along with two Super Bowl wins and a third AFC title.

Through 24 seasons, Reid is 247-138-1.

Dallas entered the NFL as an expansion team in 1960, and the Cowboys ended 0-11-1 in that first year under Tom Landry. They finished no better than 5-8-1 in the first five seasons and went 7-7 in 1965.

And then, the franchise thrived for two decades.

Landry propelled the Cowboys to 20 straight years with a winning record, and they made the playoffs in 18 of those seasons. Dallas won Super Bowls in 1971 and 1977, NFC championships in 1970, 1975 and 1978 and 13 division titles during that remarkable stretch.

Remembered for pioneering the 4-3 defense and its Flex evolution, Landry amassed a 250-162-6 record in 29 seasons.

Bill Belichick coached the Browns from 1991-95 and made the postseason in 1994, but he mustered a 36-44 mark overall.

The next portion of his NFL journey led him to the New York Jets—and a hilariously strange trivia fact. Belichick briefly served as the interim coach in 1997 before the Jets and New England Patriots figured out compensation for Bill Parcells. In 2000, he followed Parcells as the boss—and lasted a single day before accepted the Patriots' job.

Completely bizarre. Totally worked out.

Thanks to a 20-year partnership with Tom Brady, Belichick has gathered 262 wins (and counting) in New England with 17 division titles, nine AFC championships and six Super Bowl triumphs.

Belichick, who sports a 298-152 record, is poised to join George Halas and Don Shula as the only members of the 300-win club.

Prior to the 1970 merger, Don Shula coached the Baltimore Colts for seven seasons. He posted a 71-23-4 record, winning the 1968 NFL championship (though the Colts lost in the Super Bowl).

Shula left for the Miami Dolphins as the AFL and NFL joined, and the franchise immediately excelled.

Miami hadn't crested five victories in four previous years; for the first 16 seasons under Shula, the Dolphins won 10-plus games 13 times. They won 10 division titles and made five Super Bowl appearances with two victories, including the perfect 1972 campaign.

The unfortunate part of the story is Shula and Hall of Fame quarterback Dan Marino never won a ring together. Miami had only one losing record in their 13 years but also didn't make the Super Bowl after 1984, Marino's second season.

Shula retired after the 1995 campaign, holding a 257-133-2 mark in Miami and an NFL-best 328 career wins.