In American and Canadian football, the ball is punted downfield to the opposing team, usually on the final down, with the hope of giving the receiving team a field position that is more advantageous to the kicking team when possession changes.
The team in possession of the ball will typically punt the ball to the opposing team when they are on their final down (fourth down in American football, third down in Canadian football), do not want to risk a turnover on downs by not gaining enough yardage to make a first down and are in such a field position that they do not believe they can successfully make a field goal. The purpose of the punt is for the team in possession, or "kicking team", to move the ball towards the opponent's end zone in order to maximize the distance the receiving team must advance the ball in order to score a touchdown when they take possession of the ball.
A punt play involves the kicking team lining up at the line of scrimmage with the kicker, or punter, lined up usually 15 yards behind the center (in American football this distance is shortened if the ball is on a spot such that the kicker's normal position is on or beyond the end line). The receiving team lines up with one or two players downfield to catch the ball. The center makes a long snap to the kicker who then drops the ball and kicks it before it hits the ground. The player who catches the ball is then entitled to attempt to advance the ball.
The result of a typical punt, barring any penalties or extraordinary circumstances, is a first down for the receiving team at the spot where:
the receiver or subsequent receiving team ball carrier is downed or goes out of bounds;
the ball crosses out of bounds, whether in flight or after touching the ground;
there is "illegal touching", defined as when a player from the kicking team is the first player to touch the ball after it has been punted beyond the line of scrimmage; or
a ball which is allowed to land comes to rest in-bounds without being touched.
Other possible results include the punt being blocked behind the line of scrimmage, and the ball being touched, but not caught or possessed, downfield by the receiving team. In both cases the ball is then "free" and "live" and will belong to whichever team recovers it.
If the kicked ball is blocked and fails to cross the line of scrimmage, it may be picked up and advanced by either team. However, if it is picked up by the kicking team, the play is treated as any other play from scrimmage; i.e., if it is the team's final down, it must advance the ball beyond the first down marker in order to avoid a turnover on downs.
The official rules regulate when and how the receiving team may hit the kicker before, during, and after the kick.
If the receiving team drops the ball or touches the ball beyond the line of scrimmage without catching it then it is considered a live ball and may be recovered by either team. If the receiving team never had full possession, it is considered to be a muffed punt rather than a fumble. However, the receiving player must be actively pursuing the ball. If the receiving player is blocked into the ball, it is not considered "touching" the ball.
The player attempting to catch the kicked ball may attempt a fair catch. If caught, the ball becomes dead and the receiving team gets the ball at the spot of the catch.
A touchback may be called if the kicked ball enters the receiving team's end zone without first touching any player, whether in flight or on a bounce; or if the receiving team catches the ball in its own end zone and downs it before advancing the ball out of the end zone; or if the ball enters then exits the end zone. The receiving team gets the ball at its own 20-yard line.
If a player from the kicking team is the first to touch the ball after it crosses the line of scrimmage, "illegal touching" is called and the receiving team gains possession at the spot where the illegal touching occurred. This is often not considered to be detrimental to the team committing the "illegal touching"; for example, it is common for a player on the kicking team to deliberately make contact with the ball near the goal line before it enters the end zone to prevent its being ruled a touchback and placed on the 20-yard line. Since there is no further yardage penalty awarded, the kicking team is often said to have "downed the ball" when this occurs (and the NFL does not count it as an official penalty). While the ball is not automatically dead upon an illegal touch, and can be advanced by the receiving team (who would then have the choice of accepting the result of the play or taking the ball at the spot of the illegal touch), this rarely happens in practice, as illegal touching typically occurs when members of the kicking team are closer to the ball than members of the receiving team.
The length of the punt, referred to as punting yards or gross punting yards, is measured from the line of scrimmage (not the spot where the punter punts) to whichever of the following points applies: (1) the spot that a punt is caught; (2) the spot that a punt goes out of bounds; (3) the spot that a punt is declared dead because of illegal touching; or (4) the goal line, for punts that are ruled touchbacks.
The net punting yardage is taken by calculating the total punting yardage and subtracting any yardage earned by the receiving team on returns, and subtracting 20 yards for each touchback.
The kicker and any players behind him at the time of the kick are considered "onside"; any other players on the kicking team are considered "offside". A player who is onside may recover the kicked ball, while a player who is offside may not be the first to touch the kicked ball and is required to remain at least 5 yards from an opposing player attempting to catch the ball. Violations of these restrictions on an offside player are called "no yards" infractions, with various penalties associated with them.
The ball remains in play if it enters the goal area (end zone) until it is downed by a player on either team or goes out of bounds:
If a member of the receiving team downs it in the goal area or the ball goes out of bounds before being brought back into the field of play, a single is awarded to the kicking team and the receiving team gains possession at their own 35-yard line.
If an onside player downs the ball in the goal area the kicking team is awarded a touchdown.
If an offside player downs the ball in the goal area the receiving team gains possession after a "no yards" penalty is applied from their own 10-yard line.
If the ball strikes the goalpost assembly while in flight the receiving team gains possession at their own 25-yard line.
The length of the punt is measured from the line of scrimmage to the spot of the catch or the point where the kick goes out of bounds. The punt return is measured independently, though the value of the punt to the kicking team is determined by distance from the line of scrimmage to the end of the return.
Canadian rules also allow a punt when the punter is not behind the line of scrimmage, which is not permitted in American rules. This tactic (termed an "open-field kick" in the rule book) is usually reserved for last-second desperation: for example, a player, after receiving a forward pass with no time left on the clock and with no hope of evading tacklers, may punt the ball in the hope that it will score a single or be recovered by an onside teammate. After recovering a ball kicked by the other team a player can also punt out of his own end zone in order to avoid a single. On one occasion in the CFL, a last-second missed field goal attempt was followed by three punts, all on one play, as the teams alternately tried to avoid a single and score a single 
On very rare occasions, a punting team will elect to attempt a "fake punt" — line up in punt formation and begin the process as normal, but instead do one of the following:
The punter may choose to run with the ball.
The ball may be snapped to the upback, who then runs with the ball.
The punter (or another back, who is standing nearby) may decide to pass to a pre-designated receiver.
The ball may be snapped to the upback, who then passes the ball to a receiver.
Although teams sometimes use fake punts to exploit a weakness in the opposing team's defense, a fake punt is very rare, and often used in desperate situations, such as to keep a drive alive when a team is behind and needs to catch up quickly, or to spark an offense in a game where the defense dominates. The high risk of "fake punts" and the need to maintain an element of surprise when the play is actually called, explains why this play is seldom seen. Fake punts are more likely to occur when there is short yardage remaining to secure a first down, and/or the line of scrimmage is inside the opponent's territory.
Conversely, teams may line up in a normal offensive formation and have the quarterback perform a pooch punt, also known as a quick kick. This usually happens in situations where the offense is in a 4th and long situation in their opponents territory, but are too close to the end zone for a traditional punt and (depending on weather conditions) too far for a field goal try. Like fake punt attempts, these are rarely tried, although Randall Cunningham, Tom Brady, and Ben Roethlisberger have successfully executed pooch punts in the modern NFL era.
The longest punt in North American pro football history is a 108-yarder by Zenon Andrusyshyn of the CFL'sToronto Argonauts (at Edmonton, October 23, 1977). This record was also tied by Christopher Milo of the Saskatchewan Roughriders on October 29, 2011, at a home game at Mosaic Stadium in Regina, during which winds gusted above 35 miles per hour (56 km/h) (this is also the site of the three longest field goals in CFL history and one of the windiest fields in professional football).
A punt return is one of the punt receiving team's options to respond to a punt. A player positioned about 35–45 yards from the line of scrimmage will attempt to catch or pick up the ball after it is punted by the opposing team's punter. He then attempts to carry the ball as far as possible back in the direction of the line of scrimmage, without being tackled or running out of bounds. Players may also lateral the ball to other players on their team, to keep the play alive if he expects to be tackled or go out of bounds. DeSean Jackson of the Philadelphia Eagles is the only player, in the "Miracle at the New Meadowlands", to return a punt for a touchdown on the final play of a NFL game for a win from scrimmage. The NFL record for the number of punt returns for a touchdown in a career increased to 11 and is held by Devin Hester of the Chicago Bears who achieved the feat with a 69 yard return against the Carolina Panthers on October 2, 2011. He was previously tied with Eric Metcalf.