In law, trespass can be:

1. the criminal act of going into somebody else’s land or property without permission;
2. it is also a civil law tort that may be a valid cause of action to seek judicial relief and possibly damages through a lawsuit.

In some jurisdictions trespassing is an offense or misdemeanor covered by a criminal code. In other jurisdictions, it is not considered a crime or penal in nature, property is protected from trespass under civil law and privacy acts. In England and Wales, unless the trespass is aggravated in some way, it will only be a civil wrong.

Although criminal and civil trespass laws vary from jurisdiction to jurisdiction, most have the following facets in common:

• Property owners and their agents (for example, security guards) may only use reasonable force to protect their property. For example, setting booby traps on a property to hurt trespassers or shooting at trespassers are usually strictly forbidden except in extreme circumstances.
• Not all persons seeking access to property are trespassers. The law recognizes the rights of persons given express permission to be on the property ("licensees") and persons who have a legal right to be on the property ("invitees") not to be treated as trespassers. For example, a meter reader on the property to read the meter is an invitee, as would be a travelling salesperson, or a police officer seeking to execute a warrant.
• Most jurisdictions do not allow "self-help" to remove trespassers. The usual procedure is to ask the trespassing person to leave, then to call law enforcement officials if they do not. As long as the trespasser is not posing an immediate threat, they cannot be removed by force. It is usually illegal to arrest a trespasser and hold them on the property until law enforcement arrives as this defeats the purpose of allowing them to cure the trespass by leaving.
• Marking property as private property can be done in a variety of ways. The most obvious way is to put up a sign saying "No Trespassing" or "Private Property". However, a continuous fence has the same effect in most places. Many jurisdictions allow the use of markers when fencing would be impractical or expensive. For example, Ontario, Canada allows the use of red paint on landmarks such as trees to mark the boundaries of private property.
• Property owners may allow some trespasses while excluding others. For example a sign "No Hunting" would allow hiking, snowmobiling, or bird-watching, but would give notice to hunters that they would be trespassing if they entered onto the property.
• Trespass is not limited to human beings. For example, the owner of cattle or dogs may be responsible for an animal's trespass in some jurisdictions.

See also right-of-way.

The term is also used for a transgression in general, also in the Lord's Prayer.