Kirby Smart and Will Muschamp have been friends for more than two decades. But today, they will be rivals, coaching on opposite sidelines in the Iron Bowl.

Over the past 21 years, Smart and Muschamp have been teammates, colleagues and friends. They’ve coached with each other and against each other. They have followed similar paths in their coaching careers. They’re both fierce competitors. And on Saturday, they will be rival defensive coordinators in one of college football’s most intense rivalries.

Smart is the defensive coordinator at Alabama. Muschamp holds the same position at Auburn. On Saturday, they will stand on opposite sidelines and try to figure out how to stop the other team’s offense.

It will be the latest chapter in their long history.

Smart and Muschamp first met in 1994, when they were both safeties for Georgia. Muschamp was a senior co-captain and Smart was a freshman who was redshirting. Muschamp took Smart under his wing and tried to help him learn the safety position, so that he would be ready when his chance came. That Georgia team went 6-4-1, in Ray Goff’s next-to-last year in Athens. Smart would go on to play four years in the Bulldogs’ secondary, and was an All-SEC safety his senior year.

Six years later, their paths merged again. Muschamp became the defensive coordinator at Valdosta State. The team needed a secondary coach, and Muschamp recommended Smart, who was hired.

Valdosta State has a nice campus, but its football facilities were somewhat primitive compared to what Muschamp and Smart work in today in the SEC. The football offices, locker room and weight room were located inside an old basketball gym. The team played its home games in a high school stadium that was almost 80 years old, had cracks in the foundation and needed to be rebuilt. There were no windows on the coaches’ box, so if a coach got animated, the fans sitting in the stands (including the elderly school president and his wife) could hear what they said.

It wasn’t a football paradise, but it gave Muschamp and Smart a chance to show what they could do. The year before, Valdosta State had gone 4-7. With a new coaching staff that included Muschamp and Smart, the Blazers went 10-2, with both losses coming to eventual Division II national champion Delta State.

Shortly after that season, Muschamp met Nick Saban, who hired him as LSU’s linebackers coach. Smart got Muschamp’s old job as Valdosta State’s defensive coordinator.

Muschamp became LSU’s defensive coordinator in 2002 and helped the Tigers win the BCS championship in 2003.

After that season, LSU lost its secondary coach. Once again, Muschamp recommended Smart for the opening, and after interviewing him, Saban hired him. Smart lived with Muschamp and his wife for a short time in Baton Rouge. The Tigers went 9-3 that year and finished 16th in the final AP and Coaches polls.

After that season, Saban left LSU for the Miami Dolphins, and Muschamp joined him to become the assistant head coach and work with the defense. Smart became Georgia’s running backs coach. A year later, Smart joined Saban’s staff with the Dolphins, while Muschamp left to become Auburn’s defensive coordinator.

Saban left the NFL for Alabama in 2007, and took Smart with him to Tuscaloosa. The first time Smart and Muschamp faced each other was the 2007 Iron Bowl, which Auburn won 17-10.

In 2008, Smart was promoted to defensive coordinator by Saban (a position he’s held ever since), while Muschamp became the defensive coordinator and head coach in waiting at Texas.

In 2009, Smart and Muschamp got a chance to face each other in the BCS Championship Game. That night, No. 1 Alabama defeated No. 2 Texas 37-21. It was the first of Saban’s three national championships with the Crimson Tide (Smart has been the defensive coordinator for all three titles).

Muschamp returned to the SEC in 2011, when he became Florida’s head coach. The third time Muschamp faced Smart, eventual national champion Alabama routed Florida, 38-10, in 2011. Three years later, they met again, when the Crimson Tide defeated the Gators 42-21.

Muschamp was fired as Florida’s head coach at the end of the 2014 season, and was hired by Gus Malzahn as Auburn’s defensive coordinator.

In Smart’s coaching career, he has been a part of three national championship teams (all at Alabama) and four SEC championship squads (three at Alabama, one at Georgia). Muschamp has been a part of one national championship team (LSU in 2003) and two SEC championship teams (LSU in 2001 and 2003).

This season, Alabama leads the SEC in total defense, giving up 278.1 yards per game. Auburn ranks 13th, allowing 411.8 yards a game.

Saturday will be the fifth time that Muschamp has faced Smart. After the game, they’ll probably congratulate each other and might even talk for a minute, but until then, they’ll be focused on helping their respective teams win the Iron Bowl.

Auburn (6-5) hosts No. 2 Alabama (10-1) today at 3:30 p.m. ET. The game will be shown on CBS.