Trespass is the act of entering onto or using real property when there is no right of ownership, permission, or legal justification for doing so. It is both a tort and a crime.

The level of use or entry required is very low, essentially any act which is inconsistent with the rights of the owner. For example, entering another's property at all without permission or legal right is trespassing. However, it encompasses a much larger range of acts, inclusing allowing foraging animals to feed on the crops of the owner. Moreover, it allows the property owner to clearly allow compatible uses of the property and to forbid others at their will. For example, a property owner can freely allow hiking on their property while still maintaining the ability to keep out vehicles.

The idea that a property owner must post a sign saying "no trespassing" to enforce their ability to keep out trespassers is a myth. Although the law varies from place to place, the requirements for marking a piece of property as private are often very low. Usually erecting a fence is more than sufficient, but in some places merely placing bright markers at the perimeter of the property is necessary where fencing would be too expensive or impractical.

Although a property owner is free to secure their property from trespassers, there is a limit to the lengths that a person may use to do so. These limits also vary from place to place. In many jurisdictions, the owner or their lawful agent (a caretaker or security guard for example), may invoke those rights merely by asking a trespasser to vacate the property. If the trespasser refuses to do so, the owner is within their rights to use reasonable force to expel the trespasser, or may be allowed to call law enforcement officials to remove the trespasser. However, a property owner may not lay a trap against trespassers, and may not threaten and use deadly force (i.e. "all trespassers will be shot") to protect their property rights.

See Trespass to chattels for a similar tort with respect to personal property.

See also Adverse possession