In rugby union, you can only tackle a player in possession of the ball.
Sometimes, a player will deliberately get in the way of an opponent because they think their rival has a good chance of getting to the ball first, but it is not allowed.
You cannot charge or push an opponent when running for the ball, except if the contact is shoulder-to-shoulder.
Blocking an opponent in order to stop them tackling one of your team-mates, or moving in front of a team-mate in order to act as a shield, is also outlawed.
Standing in a position which stops an opponent from playing the ball is also considered to be obstruction.
Players running with the ball after it has left a set-piece cannot make contact with a team-mate in front of them.
And flankers cannot block the opposition's scrum-half as they try to advance around the scrum.
For all these offences, a penalty is awarded to the opposition.
The offender could also find themselves shown a yellow card, which results in a 10-minute spell in the sin-bin.