Checking is the act of taking an opposing player away from the puck by means of body contact.
A check is legal as long as the player being checked has the puck or is close enough to immediately play the puck. It is illegal to hit or check a player that does not have the puck or is not close enough to play the puck.
Checking is only allowed on the trunk of the body such as the torso, chest, or shoulder. Checking below the waist or above the shoulders is illegal. Common penalties for these illegal hits include kneeing, head contact, and roughing. Some of these are automatic major penalties and could result in fines and suspensions.
In recent years, checking from behind has also been redefined. It has been decided that blind side checks that could injure players are not beneficial to the future of the sport. Although some referees do not call all checks from behind, they do make an effort to penalize players who check from behind in a reckless manner.
In the NHL as well as other hockey leagues, checking is tough to call consistently as every situation is different. One legal hit may be called illegal by a different referee, in a different game. Some hits that are ruled as a clean and legal check can still cause injury. The sport is rough and players are taught to always know who is around them at all times. If a player is close enough to the puck, they need to be aware of possible situations where they can be hit.