After their arrival in the United States, Béla and his wife Márta Károlyi were credited with transforming the coaching of gymnastics in the U.S. and bringing major international success.

Károlyi coached many notable national, European, Olympic gymnasts as well as those from the World Gymnastics Championships including Nadia Comăneci, Ecaterina Szabo, Mary Lou Retton, Julianne McNamara, Betty Okino, Teodora Ungureanu, Kim Zmeskal, Kristie Phillips, Dominique Moceanu, Phoebe Mills, and Kerri Strug.

[2] Skilled as an athlete, he became a national junior boxing champion and a member of the Romanian hammer throwing team.

[3] He enrolled at the Romania College of Physical Education, studying and practicing gymnastics after having had trouble with a mandatory skills test in the sport.

[4] During his senior year at the college, Károlyi coached the women's gymnastics team, whose star was Márta Erőss.

He worked as a coach at the boarding school in Gheorghe Gheorghiu-Dej (now named Oneşti), training young girls specially chosen for their athletic potential.

[5] At the 1976 Summer Olympics in Montreal, Canada, he was the head coach of the Romanian squad; most of the members of the team were Gheorghe Gheorghiu-Dej athletes.

Romanian Federation officials criticized Károlyi because of his score protests at several international meets, including the 1980 Olympics.

During a 1981 gymnastics tour, Romanian team choreographer Géza Pozsár and the Károlyis defected and sought political asylum in the US, temporarily leaving their seven-year-old daughter Andrea with relatives in Romania.

ABC television network commented on this controversy during its broadcasts and often showed Retton and McNamara running over to the stands to speak to Károlyi.

[9] Károlyi's star gymnast Kristie Phillips competed in the meet finishing second behind Sabrina Mar, who trained with former Olympic coach Don Peters at SCATS gym in Huntington Beach, California.

Béla Károlyi told the United States Gymnastics Federation that he would not attend the 1988 Seoul Olympic Games unless he was the team coach.

The five gymnasts were Phoebe Mills, Brandy Johnson, Chelle Stack, and the two team alternates Rhonda Faehn and Kristie Phillips.

Phillips told reporters it would hurt the national team if they had to leave Károlyi's gym and train with Peters at SCATS.

East German gymnastic official Ellen Berger raised a valid objection, since a U.S. team member had violated one of the obscure competition rules.

After the springboard had been used at the start of another gymnast's uneven bars routine, the U.S. alternate, Rhonda Faehn, had pulled it away but had stayed on the podium to watch, instead of stepping off again immediately, as required by the rule.

An incensed Karolyi said the rule was invoked in order to "keep the scores down" because the East German team was "fighting desperately to keep their place".

[15] After the 1988 Olympics, Károlyi's sphere of influence continued to grow as did the number of elite gymnasts training in his gym.

The situation was nearly repeated at the 1992 Olympics, where Károlyi was head coach and five members of the seven-gymnast squad (six competitors and one alternate) were either trained by him or one of his protégés.

Károlyi primarily was a personal coach for Dominique Moceanu and Kerri Strug at the 1996 Olympics, but he still managed to draw the spotlight.

While American gymnasts did win medals in international competitions such as the Goodwill Games and the Pacific Alliance Championships, they were largely unsuccessful in most major meets.

After the 1999 World Championships, USA Gymnastics tried to revamp its program by hiring Károlyi as national team coordinator.

Károlyi required that all national team members attend frequent, grueling camps at his ranch north of Houston.

Some of his former athletes including Kristie Phillips, Dominique Moceanu, and Erica Stokes have publicly said that Károlyi was verbally and psychologically abusive during workouts.

Károlyi's constant critical remarks about weight and body type were said to drive some gymnasts to develop eating disorders and low self-esteem.

[24] Some gymnasts, such as Phillips, Moceanu, and 1988 Olympian Chelle Stack also said that they were compelled to continue training and competing even when coping with serious injuries such as broken bones.

Nadia Comăneci, in her 2004 memoir Letters to a Young Gymnast, remarked that she literally trusted Károlyi with her life.

She also stated that in Romania, the gymnasts at Károlyi's school consumed well-balanced diets and, in fact, ate better than most other civilians in the country at the time.

In November 2008, Emilia Eberle, a Romanian national team member during the Károlyi coaching era, gave an interview to KCRA-TV in Sacramento, California, claiming that while she was on the national team, both Béla and Márta Károlyi regularly beat her and her teammates for mistakes they made in practice or competition.

Nassar reportedly groomed athletes for abuse and gained their trust in part by covertly providing them with food in defiance of Károlyi's strict dietary guidelines.