Tackling remains one of the fundamental skills in football, as it occurs on nearly every play. Rules exist to ensure the legality of all tackles performed during a game. Football players have the responsibility to tackle opponents in a legal manner, and the league's officials have the responsibly to enforce these rules at all times.
In football, defensive players must stop the offensive team by tackling the ball carrier. They can do so in almost any manner, although some exceptions do exist. When making a tackle, the defensive player can grab his opponent's jersey or body in an attempt to stop his forward process. This includes grabbing the player's legs to trip him or hitting him with your shoulder. The play stops once the defensive player has the offensive player on the ground or has stopped the offensive player from moving forward.
Offensive players cannot tackle players on the defense, unless the defensive player first gains possession of the ball. An offensive lineman, for example, can use his hands to keep the defensive player in front of him, but he cannot drag that player to the ground, or the offensive player will be flagged for holding. This penalty usually occurs as a defensive player approaches the quarterback, as the offensive lineman must do everything in his power to prevent a sack. Offensive holding results in a 10-yard penalty.
In some cases, players tackle opponents dangerously, which leads to a penalty. A tackler receives a 15-yard penalty for unnecessary roughness for leading with his helmet; for hitting an offensive player in the head during a tackle; for tackling a player who's out of bounds; or for tackling a player after the whistle has blown. Defenders receive 15-yard penalties for roughing the passer for tackling a quarterback after he throws the ball. Defensive players are penalized 15 yards if they grab the back of a player's shoulder pads to make a tackle -- known as a "horse collar" tackle. Players receive either a 5- or 15-yard penalty if they hold the facemask of a player's helmet when making a tackle.
The NFL has implemented new tackling rules to protect its players, which led to the league handing out a number of fines. Linebacker Jerome Harrison threatened to retire in 2010 because of these rules, since he did not know if he could still play the game effectively. The league fined Harrison for tackles that did not receive a penalty during the game, which blurs the line between legal and illegal tackles. These tackles involved Harrison striking opposing players in the head with his helmet, causing injuries to two different players in one game.