Chronic traumatic encephalopathy is a hot button issue in sports these days. Experts have known about it for over a century, but it was only recently that there came to be a serious public outcry over athletes suffering from mental issues later in life.

In the most basic terms possible, for those still unfamiliar with the condition: CTE comes from too many blows to the head. It doesn't depend on how many diagnosed concussions a person has, but concussions are obviously an issue. Every time an athlete sustains a blow to the head, whether or not they sustain a diagnosable concussion, there is risk for the proteins that appear in the brain due to CTE to develop.

Decades of this buildup can cause severe mental conditions such as depression, dementia and these can lead to irrational and erratic outbursts and even suicidal behavior. While CTE is most common among football and hockey players, many boxers have been noted as having the condition and even baseball players have been diagnosed with CTE type degenerative conditions.

This is a list of the most well-known instances of athletes having serious brain issues and their actions as a result of CTE. There is some disagreement among professionals, as to which athletes have CTE. Occasionally, athletes who died ages ago are speculated to have had the condition but in some cases there is no way to know.

Some very well known athletes will not be found on this list; including Muhammad Ali, Mike Tyson and Lou Gehrig. These three former athletes have all been suggested to have had the condition, but do not make this list for various reasons.

Some researchers have suggested that while Gehrig had Amyotrophic Lateral Sclerosis (ALS, also known as Lou Gehrig Disease), he may have also had CTE, which could have sped up his degeneration. Many note the fact that Gehrig was a multi-sport athlete for much of his life and sustained many head injuries prior to his baseball career. He did not make the list because the evidence of his having CTE is just to vague and speculative.

Muhammad Ali is not here because, despite many experts suggesting he may have CTE, his doctors all insist that he suffers from Parkinson's that did not result from the condition.

Finally, while some have joked and commented that "Iron" Mike Tyson has the condition; most dismiss the theory. Many doctors think that Mike Tyson's personality is a result of something else altogether.

The fight to treat and prevent CTE is underway, but as of right now, there is very little in the way of evidence that the battle will be won or lost anytime soon. Sports leagues are cracking down on hits to the head area, but fines and suspensions are not stopping this type of contact. This list is ordered from the speculated, minor and self-diagnosed cases of CTE to those that are extremely notorious and proven.

20 20. Brett Favre

After many amazing years with the Green Bay Packers followed by a few passable years with the New York Jets and Minnesota Vikings, Favre still holds the record for most consecutive starts by an NFL player. Unfortunately, with so many games under his belt, he sustained many knockdowns and tackles. In October 2013, he revealed that he was dealing with memory loss, which prompted many experts to speculate that he is suffering from a minor case of CTE.

19 19. Bob Probert

Possibly the NHL's best enforcer of all time, Bob Probert, died back in 2010. Months later, when his brain was examined by researchers at Boston University (one of the leading CTE research centers in the United States), they found evidence that he was suffering from CTE. Probert had legal and substance problems after his retirement from the NHL, and some have argued that his erratic behavior was due to brain trauma.

18 18. Ollie Matson

Ollie Matson was not only an NFL running back but also a bronze medalist sprinter who competed in the 1952 Olympics in Finland. Matson played 17 seasons in the NFL, making the Pro Bowl six times and being inducted into the Pro Football Hall of Fame. He died back in 2011 of dementia and was later revealed to have suffered from advanced CTE.

17 17. Bernie Kosar

Bernie Kosar was an NFL quarterback from 1985 to 1996, during which he won a Super Bowl with the Dallas Cowboys as a backup and was a Pro Bowl selection in 1987. Kosar dealt with multiple concussions throughout his career and has suffered pain and mental issues ever since his retirement. In recent years he has received treatment and advocated methods of treatment for other players who have developed CTE.

16 16. Sugar Ray Robinson

One of the most decorated boxers of all time is also argued by many to have suffered from serious CTE later in life. "Sugar" Ray fought a total of 198 professional bouts between 1940 and 1965, winning 173 of them; many by knockout. While he lived a very lavish lifestyle while fighting, he suffered financially later in life and developed Alzheimers before dying in 1989 at just 67.

15 15. John Mackey

Mackey was one of the most dominant tight ends of his time, playing nine seasons for the Baltimore Colts, including two NFL Championships and a Super Bowl. The five time Pro Bowl selection and 1992 inductee to the Hall of Fame suffered from advanced CTE later in life, until his death in 2011. His wife sued the NFL after his death. Mackey was one of the players whose brain degeneration prompted the NFL's "88 Plan," named for his jersey number. This plan essentially put aside aid money for retired NFL players suffering from degenerative diseases who required full time care.

14 14. Joe Louis

Another one of boxing's heroes, former world champion Joe Louis is now largely believed to have suffered from CTE in his later years. Participating in 70 fights between 1934 and 1951, he won 66 and is considered one of the first athletes to challenge the racial barrier in the United States. Known for his ability to soak up damage in the ring, Louis was known for being "punch drunk" in his later years. "Punch Drunk" was the layman's term for CTE-like conditions suffered by boxers after their careers. He suffered well-documented physical and mental breakdowns throughout his post-boxing life and these have been attributed to severe mental degeneration and drug use.

13 13. Reg Fleming

Playing 23 years in the NHL for six different teams, Reg Fleming was a tough player who played from the mid 1950's to the 70's. He was an aggressive player that took nonsense from nobody, but sadly, he suffered from advanced CTE later in life. He died in 2009 and shortly thereafter, Boston University researchers determined that he had suffered from the condition for decades. Fleming was the first known hockey player to have the condition.

12 12. Andrew "Test" Martin

Professional wrestlers take as many hits to the head as any other group of athletes. WWE wrestler Andrew "Test" Martin was a member of the promotion from 1998 until 2008. Just a year later in March 2009, he was found dead in his apartment from an oxycodone overdose. After his death, doctors determined he had suffered from CTE. He was the second wrestler to be diagnosed with the condition post-mortem.

11 11. Dave Duerson

Dave Duerson was a safety who played in the NFL for eleven years before retiring. He won a Super Bowl with the Chicago Bears in the 80s and another with the New York Giants in the late 90's. Duerson committed suicide in February 2011, and prior to his death, he texted his family indicating that he wanted his brain examined and used to study CTE and concussion trauma. The four time Pro Bowl selection was 50 years old when he took his own life.

10 10. Chris Henry

Drafted in 2005, Cincinnati Bengals' wide receiver Chris Henry had a rough career that was as riddled with controversy as it was short. He was accused of assault, had multiple arrests for illegal substance and gun charges. In 2009, at age 26, Henry was killed in a car accident. Researchers later discovered that his brain showed signed of CTE. At that point he was among the youngest people to have suffered from the disease.

9 9. Derek Boogaard, Wade Belak and Rick Rypien

In the spring and summer of 2011, Derek Boogard, Wade Belak and Rick Rypien all died. All were tough guys in the NHL. Rypien and Belak committed suicide while Boogaard died of a substance overdose. All three had a history of concussions, and while Boogaard was the only one to have had full blown CTE, the other two were suspected of having suffered from the condition, given their chronic depression and history of concussions.

While these three cases all had significant differences, their close time frame made them notorious in the hockey world and reignited a debate similar to that over brain health in the NFL.

8 8. Andre Waters

The hard hitting safety for the Philadelphia Eagles and Arizona Cardinals during the 80's and 90's was found dead in his home in 2006. This was approximately ten years after his retirement from playing. He committed suicide via gunshot and after his brain was examined by researchers at the university of Pittsburgh he was determined to have suffered from depression due to CTE.

7 7. Mike Webster

One of the greatest offensive linemen of all time, Mike Webster was a nine time Pro Bowl selection, a nine time All-Pro and played on four of the Pittsburgh Steelers' 1970's Super Bowl winning teams. He suffered from severe CTE after his playing career, experiencing depression, dementia and staggering pain. Webster died in 2002, and was the first NFL player to have been diagnosed with CTE. Divorced by his wife and spending periods of time homeless, Webster's later years were a sad state of affairs for a great athlete.

6 6. Owen Thomas

While Chris Henry seemed young at just 26 when he was found to have suffered from CTE, Penn State offensive lineman Owen Thomas was just 21 when he was found in his apartment. He had committed suicide, due to an emotional breakdown that many around him called uncharacteristic. Thomas' brain was examined by specialists and he was found to have been suffering from CTE.

5 5. Ryan Freel

Ryan Freel is the first baseball player to be diagnosed with CTE. He played nine years in MLB and retired after a series of injuries. While playing center field he sustained a serious head injury while chasing a fly ball. Just months later he took a ball to the head and sustained a concussion.

Freel retired in 2009 but had sustained significant brain trauma. He committed suicide in 2012 by gunshot wound. Examiners determined that he had suffered from CTE due to his baseball injuries.

4 4. Nathan Stiles

The youngest deceased player to have suffered from CTE, Stiles was in high school when he died. Stiles was a running back playing in his senior year for his high school, when he sustained a serious hit to the head, and later died from second-impact syndrome. Basically, he sustained head trauma too soon after sustaining an initial concussion.

During his autopsy, doctors revealed that Stiles, just 17 years old at the time of his death, was already showing signs of CTE. This occurred back in 2012.

3 3. Junior Seau

One of the greatest linebackers of all time, Junior Seau recorded over 1800 tackles, and almost 60 sacks throughout his career. He played twenty seasons in the NFL, for the San Diego Chargers, Miami Dolphins and finally the New England Patriots. Seau was a twelve time Pro Bowl selection. Seau committed suicide via gunshot wound to the chest back in 2012. His family claimed that although he rarely missed time due to injury or concussions during his career and there was no doubt among them that he had sustained head trauma throughout. For those who remember his career, he was the definition of toughness and resilience on the field.

Doctors determine that his brain showed significant signs of CTE. He suffered from mild depression and severe sleep disorders for years before his death. Junior Seau was just 43 years old.

2 2. Jovan Belcher

On the weekend of December 1st, 2012, NFL fans learned that Kansas City Chiefs' linebacker Jovan Belcher had shot his girlfriend and then himself, leaving behind a young daughter. He was just 25 years old. Over a year after the incident, Belcher's family had his remains exhumed and requested his brain be examined for signs of serious brain trauma. While the results of these tests were not released to the public, many speculated that mental illness may have played a part. In late September of 2014, several reports were released stating that Belcher had indeed suffered from CTE.

1 1. Chris Benoit

Chris Benoit was a Canadian wrestler who competed from the mid-1980's until 2007. In 2007, he murdered his wife and son, later committing suicide by hanging himself in his home gym. Benoit's horrific actions caused re-ignition of a couple of debates. The first of these was regarding the use of steroid and other PED's, to which many attributed his behavior. Others have claimed that Benoit had a drinking habit that more likely led to his actions.

Examination of his brain, however, led to discussion of brain trauma in professional athletes. To paraphrase the examining doctor, he had the brain of an 80 year old, and had sustained lasting damage to every section of his brain. Benoit had one of the most severe cases of CTE that examiners had ever seen. Benoit was only 40 years old.