The ball is lighter, the players are faster, the tactics are more complex. And if you’re a referee working the 2010 World Cup and you can’t keep up and be in the right position, you may blow the call, outraging the hundreds of millions of fans watching worldwide.

So officials working the games have to be quicker and fitter. And it means they spend long days scouting the teams, just like opponents scout each other, so they can anticipate the action and make the right call.

“Teams have a reputation for a style of play. Players have certain tendencies,” says Paul Tamberino, U.S. Soccer’s director of referee development. “The referees need to know those going in, to know what to expect. African teams are extremely speedy. So you prepare your assistant referee (who calls offsides) to play the offside line. Germany is good on free kicks in the offensive third and very good on head balls. So you need to be ready for contact inside the penalty area.”

For officials, the road to the World Cup is as competitive and demanding as it is for players. Referee and assistant referee candidates have their fitness monitored monthly in the three years leading up to the Cup. They meet with a psychologist who analyzes their game demeanor. They attend seminars on the rules in an attempt to apply them equally across every continent where soccer is played. They go online to a virtual classroom to discuss their doubts and concerns with instructors and colleagues.

Those components are part of FIFA’s Refereeing Assistance Program, implemented in 2007 to improve officiating and respond to criticism. Fifty-four trios of officials went into the program and had their performances at FIFA tournaments evaluated. Thirty of them were chosen this February to work the World Cup. “Each referee has had to prove his ability out on the pitch,” said José María García-Aranda, head of FIFA’s Refereeing Department.

Referee controversy has long been a part of the World Cup. In 1986, a Tunisian refereeing his first Cup game—England against Argentina in the aftermath of the Falklands War—failed to notice the Argentine Diego Maradona punch a ball with his hand past a stunned English goalkeeper. While the early games of the 2010 World Cup were without problems and analysts praised the refereeing, that ended with the match pitting the United States against Slovenia, a 2-2 tie. Late in the game, a Mali referee, Kouman Coulibaly, working his first World Cup game disallowed a goal by U.S. player Maurice Edu. The U.S. team complained that he refused to cite a reason for the decision, although later he said it was for a foul by Edu. Replays showed no foul.

At the last World Cup, a Russian referee issued 16 yellow cards and 4 red cards, matching a record. FIFA President Joseph S. Blatter said the referee should have given himself a yellow card for his poor performance, then backed off his remarks and apologized.

In another game, a British referee cautioned the same player three times before sending him off the pitch (rules require a player to be ejected after two yellow cards). He later retired from international-tournament refereeing, citing the error.

For the World Cup, trios of match officials are chosen as a team. They are the referee on the field, who controls the game and calls fouls, and two assistant referees on the sidelines, who mainly call offsides and determine who gets possession after the ball goes out of bounds.

They have to prove their fitness on the pitch and in the lab. Among the fitness tests are two showing whether referees can run with players, some half their age. For the first test, a referee runs 40 meters six times. Each of the six sprints needs to be completed in 6.2 seconds. A second test requires a referee to run 150 meters in 30 seconds and then walk 50 meters in 35 seconds, then repeat the exercise 19 more times. Assistant referees have less strict standards. To track their fitness, referees constantly wear a watch that monitors their heart rates.

In the lab, a medical team in Zurich assessed each match official earlier this year. Among the tests were a blood test, an orthopedic examination, a resting EKG, an echocardiogram and a stress test.

In late May, FIFA announced that 2 of the chosen 30 teams would not be officiating at the World Cup because an assistant referee in each team had failed the final fitness test.

FIFA says that because of the huge pressure on match officials, sports psychologists help each of them develop a personalized strategy to cope with it and prevent it from affecting their work and personal lives.

Instructors maintain close contact with match officials throughout the World Cup games to discuss any concerns. Before games, match officials meet to discuss the problem players, the matchups, the coaching philosophies and the consequences of the game. If a player is sent off early for a red card, will their team play for the tie or continue to attack because they need the point to move on to the next round?

“Referees have to be prepared,” Tamberino says. “There’s so many styles, so many tactics.”

Tamberino, named the Major League Soccer referee of the year each year from 1998 to his retirement in 2001, worked nine World Cup qualifying matches. He says the two biggest changes in the game in the past decade are the increases in speed and technical ability. “Everything is geared to make the game faster and more exciting, not that it wasn’t exciting ten years ago,” he adds. “It puts more demands on referees.”

Teams are more likely to move the ball quickly from the defensive third into the offensive end on the foot of a speedy attacker, making fitness imperative for referees, who may run seven to nine miles during a game, as much as a midfielder.

Players are also more likely to take a dive, fake being tackled. “The referee needs to be so close to see if it’s a dive or it’s really contact,” Tamberino notes. “Everything rides on that one call.”

Ultimately, for referees, it’s a game of angles, just like so many other sports. If the referee is in the right position with the right angle, he has a much better chance to make the right call.

As a referee, Tamberino believed in the players. “You want to contribute to the entertainment,” he says. “You want to let the players exhibit their skills without over-calling the game for trifling offenses, as they say in the rule book. You want to make it as enjoyable as possible for the spectators, minimize the whistles and encourage fair play.”

For a long time, Tamberino’s motto has been “nothing dirty, nothing cheap.” “We let you play hard and physical, but nothing dirty, nothing cheap,” he adds. “That’s a successful game to me.”