In various sports, a professional foul is a deliberate act of foul play intended to bring about an advantage for the perpetrator. Professional fouls are usually committed to prevent an opponent from scoring.
The most common professional foul in Association football is blocking a player in an advantage position that is not yet close to the goal. Those fouls are usually punished by just a Yellow card, and are considered a fairly advantageous tactic.
However, when the fouled player is in a very advanced position, the penalty is harsher, making it an inferior strategy, yet it is still sometimes the correct play. 
Should a defending player consider that an attacker is more likely to score from the current playing position than from a free kick or penalty, they may calculate it to be advantageous to commit a foul. Such fouls are sometimes referred to as "last man" fouls, referring to the last member of the defending side in a position to stop an attacking player having an obvious goal-scoring opportunity. This is typically the defender in front of the goalkeeper but it can be the goalkeeper. However, the terminology 'last man' is no longer included in the Laws of the Game. The offence is informally known as DOGSO, an acronym for "Denying an Obvious Goal-Scoring Opportunity". An "obvious goalscoring opportunity" means that the attacker has the ball at or near his feet, is close to the goal area and is moving towards the goal, and has no more than one defender or the goalkeeper in his way. These are known by referees as the "4 Ds," and only a foul that has all four must result in a red card.
Even with the penalty of a red card for perpetrators, deliberate professional fouls still occur in the game. And in some cases it is even considered the correct play.
The concept gained notoriety in association football after an infamous incident in the 1980 FA Cup Final when Willie Young of Arsenal committed a deliberate foul on Paul Allen of West Ham, when Allen had a clear run at goal. As the Laws of the Game stood, the referee (George Courtney) could only award West Ham a free kick, which he did. This provoked a national debate on deliberate fouls that denied opponents the chance to score a goal. At the time, the English game was suffering a downturn in attendances and the chairmen of the Football League clubs decided to consider ways in which the game could be made exciting. A subcommittee was appointed to produce some suggestions, chaired by Jimmy Hill and including Matt Busby and Bobby Charlton.
The sub-committee produced several suggestions, including making the professional foul a mandatory red card offence, which they submitted to the IFAB for consideration. All the suggestions were defeated. However, the Football League was determined to have their way, and instructed its referees that professional fouls (including deliberate handball to stop a goal being scored) should be deemed serious foul play, which was and is a mandatory red card offence. The new interpretation was first issued to referees prior to the 1982-83 season.
FIFA first instructed its referees to send off for a professional foul prior to the 1990 World Cup, and in 1991 IFAB added decisions to the law which provided that a player who committed a foul or handling offence that denied an obvious goal-scoring opportunity should be sent off for serious foul play. These decisions were incorporated in the laws in 1997.
In 1998 a tackle by Ole Gunnar Solskjær, playing for Manchester United at Old Trafford, who ran from within the Newcastle penalty area almost the length of the pitch to run down and trip Newcastle United's Rob Lee - with a clear goalscoring opportunity one-on-one with United goalkeeper Raimond van der Gouw. Solskjær was immediately sent off by referee Uriah Rennie (with the match finishing 1–1 and prolonging the Premier League title race with Arsenal).
In the 2006 UEFA Champions League Final, Arsenal's Jens Lehmann brought down Barcelona's Samuel Eto'o, then Ludovic Giuly tapped into an empty net. However, referee Terje Hauge overruled the goal, awarded Barcelona a free kick on the edge of the box, and showed a red card to Lehmann. According to Arsenal, the general consensus was that the goal should have stood and that the Gunners continue the game with eleven players.
Another controversial situation occurred during the 2010 FIFA World Cup quarter-final between Uruguay and Ghana. In extra time with the match tied 1–1, Uruguay's Luis Suárez committed a deliberate goal line handball save from Ghana's Dominic Adiyiah in the last minute of the second extra time period. As per the laws of the game, the referee issued Suárez a straight red card and awarded a penalty kick to Ghana, but Asamoah Gyan missed on the penalty kick, leaving the game level at the final whistle. Uruguay went on to win 4-2 in the shootout, and Suárez' teammates carried him around the pitch as a hero. Suárez afterward said, "The 'Hand of God' now belongs to me. Mine is the real 'Hand Of God'. I made the best save of the tournament." He was banned for the next match, although some have argued for and against lengthening the suspension. Ghana's John Pantsil argued that the referee should have allowed the goal to stand instead of pointing to the spot, stating, "In the same situation [as Suárez], there is no chance the Ghana players would have used our hand." Since Suárez's handball offense was committed at the end of extra time, there was no further period of play where his team was handicapped by being reduced to ten men, as opposed to if he had received the red card earlier in the match.
In basketball, teams may deliberately commit personal fouls for strategic reasons. As the resulting free throws must be taken by the fouled player, teams may tactically choose to foul a player with a poor free-throw percentage. This became known as "Hack-a-Shaq" after Shaquille O'Neal who was a target of such tactics.
The professional foul in rugby league embodies a similar concept, a deliberate breach of the rules in order to prevent a scoring opportunity. The penalty for this offence is 10 minutes in the sin bin.
The majority of professional fouls are either holding down the tackled player after a break has been made in order to allow his teammates to reform in defence, interfering in the play when making little or no attempt to return to an onside position, or tackling or impeding the progress of a player not in possession when a try may possibly be scored. The latter situation may result in a penalty try.
Law 10.2a of the Laws of Rugby deals with intentional infringements. Referees are instructed to award a penalty kick in such instances and admonish, caution (resulting in a temporary suspension from the game), or send off the offender. A penalty try can be awarded if the referee believes the offence probably prevented a try being scored.
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"In a close match, with seconds ticking down and a team being down by one or two points, a coaching strategy could be to foul and stop the clock and make the other team earn their victory by way of the free throw."
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