Injuries are a part of life in the NFL. Teams and players do everything they can to plan and prevent them, but you never know when, or where, that career-ending hit is going to come from.
Some players had great careers cut short by a string of injuries, while others had just begun to show their potential when the wrong hit ended everything prematurely.
Here are 10 players whose careers ended too soon.
Green Bay Packers wide receiver Robert Brooks was one of the game's elite wide receivers in the 1990s.
Then a gruesome knee injury in 1996 that tore his ACL and his patellar tendon changed his running mechanics, and caused him severe back problems.
He retired after the 1998 season, but attempted a comeback in 2000 with the Denver Broncos before retiring for good.
Defensive lineman Dennis Byrd was turning into one of the best linemen for the New York Jets when disaster struck.
In a 1992 game versus the Kansas City Chiefs, Byrd collided with teammate Scott Mersereau, crushing two vertebrae and causing him to be temporarily paralyzed.
He recovered, but his NFL Career was over.
Marlin Jackson signed with the Philadelphia Eagles this past March after starting his career, and winning a Super Bowl with the Indianapolis Colts.
Jackson had just come off back-to-back reconstructive knee surgeries, and was slated to be the starting free safety for the Eagles, when he ruptured his Achilles tendon in OTAs this past week.
Jackson had been a first-round pick by the Colts in 2005, but a third season-ending injury in as many seasons will end up leaving a good part of his career in a "could've been" scenario.
While Troy Aikman did retire at the age of 34 with three Super Bowl rings on his fingers, he retired because of the 10 concussions he suffered in his 12-year career.
With his legs and arms still relatively healthy, it's not unreasonable to assume Aikman might have been able to win a fourth Super Bowl if he hadn't been continually concussed.
Darryl Stingley had 110 receptions for 1,883 yards and 14 touchdowns in four seasons for the New England Patriots. He also had 28 carries for 244 yards and two touchdowns, 19 punt returns for 136 yards, and eight kickoff returns for 187 yards.
He had just agreed on a new contract with the Patriots when he took the now-infamous hit from Oakland Raiders legend Jack Tatum in a 1978 preseason game that left Stingley confined to a wheelchair for the rest of his life.
Center LeCharles Bentley was the big free agent signing of the 2006 offseason for the Cleveland Browns.
Bentley was supposed to anchor the Browns line for the next several years, but he ruptured his patellar tendon on the very first snap of the very first practice of the Browns 2006 training camp.
Two years, four surgeries, two staph infections, and one leg amputation threat later, Bentley was released and eventually retired after the 2008 season.
Running back Robert Edwards became one of those players more famous for his injury than for his on-field accomplishments.
Edwards was drafted by the New England Patriots in 1998 and after rushing for 1,115 yards in his rookie season, suffered a freak knee injury while playing flag football in an NFL-sanctioned Pro Bowl Week event.
The injury was so bad he almost lost his leg, and it took three years to rehabilitate. He returned in 2002 with the Miami Dolphins, but has played in the CFL the last several years.
He currently is a free agent.
Sterling Sharpe became yet another Green Bay Packers wide receiver whose career was ended prematurely by injury. Sharpe injured two of his vertebrae at the end of the 1994 season and was forced to retire.
Quarterback Brett Favre still was developing at the time, and there always has been the "what if" surrounding his career ever since.
It's been speculated Sharpe could've challenged Jerry Rice's numbers had he been able to continue playing.
Bo Jackson became one of the most famous multi-sport athletes ever. His careers with the Kansas City Royals and the Los Angeles Raiders both were headline-worthy.
During his four seasons in the NFL, Jackson rushed for 2,782 yards and 16 touchdowns, with an average yards per carry of 5.4. He also caught 40 passes for 352 yards and two touchdowns.
Then there was the hip injury versus Cincinnati on Jan. 13, 1991. The injury ended his football career immediately, and his baseball career a few years later, as his hip had to be replaced due to the degenerative nature of the injury.
Gale Sayers was on his way to challenging Jim Brown for the title of "Best Running Back Ever," when injuries to both knees ended his career after seven years.
Sayers had 22 touchdowns and 2,272 all purpose yards in his rookie season in 1965, including six touchdowns in one game.
Sayers made four Pro Bowls in his career, but his first knee injury in 1968, a gruesome injury that tore all the ligaments, robbed him of his lightning fast breakaway speed.
A second injury to his other knee in 1970 took what was left and Sayers retired in 1971.