|This article does not cite any references or sources. (August 2009)|
- This article is about the common law concept of a trespasser; for the computer game, see Jurassic Park: Trespasser
|Part of the common law series|
|Liability and remedies|
|Duty to visitors|
|Other common law areas|
In the law of tort, property, and criminal law a trespasser is a person who commits the act of trespassing on a property, that is, without the permission of the owner. Being present on land as a trespasser thereto creates liability in the trespasser, so long as the trespass is intentional. At the same time, the status of a visitor as a trespasser (as opposed to an invitee or a licensee) defines the legal rights of the visitor if they are injured due to the negligence of the property owner.
Trespassing as a tort 
Intent required 
The intent need not be to commit a trespass, but merely to go to a specific geographic place - if a person walking in a public park errantly leaves the park and enters private property, they are liable for trespass, even though they did not know that they had entered private land. However, a person who ends up on land where they did not intend to go is not liable for trespass. For example, a person walking in a public park who trips and rolls down a hill will not be liable for trespass just because the bottom of the hill is on private land.
Physical invasion 
The trespasser need not enter the land in person. Throwing any physical object onto the land of another - a rock, a clod of dirt, a bucket of water - is a trespass. Indeed, if A and B are standing next to C's land, and A pushes B onto the land without entering it himself, it is A (and not B, who did not intend to enter that space) who is liable for the trespass to C's land. There must be some physical entry, however. Causing noise, light, odors, or smoke to enter the land of another is not a trespass, but is instead a different tort, nuisance.
For purposes of determining liability, the landowner's property rights extend above and below the land to as much distance as the landowner can beneficially use. Even a low-flying plane can trespass if it enters this usable space.
Constructive trespass 
A constructive trespass occurs when a person who has permission to be on the land overstays their welcome. A person who stays in a business after its closing time, or who goes to a dinner party but refuses to leave long after the other guests have gone home, is a trespasser despite his initially proper presence. Furthermore, a guest's status as a trespasser arises as soon as he resists the property owner's command for him to leave the property. This is not a constructive trespass if the guest is unconscious.
Duties to trespassers 
With respect to the duties owed to trespassers, there are two types of trespassers to consider. First, there is the undiscovered trespasser, to whom the property owner owes only a duty not to "trap" or wilfully harm the trespasser. At the outset, the concept of traps was narrowly defined. More recently, courts have engaged in some creativity, adopting a broader interpretation of a trap.
Second, there is the anticipated or discovered trespasser. To those parties, the landowner owes a duty of common humanity (See British Railways Board v. Herrington)—a duty to warn them of deadly conditions on the land which would be hidden to them, but of which the property owner is aware. A warning sign at the entrance to the land will suffice for this purpose. However, a property owner is under no duty to ascertain hazards on his property, and cannot be held liable for failing to discover a deadly hazard which injures a trespasser.
Furthermore, an adult trespasser who is injured while on a defendant's property cannot sue under a theory of strict liability, even if the landowner was engaged in ultrahazardous activities, such as the keeping of wild animals, or the use of explosives. Instead, the trespasser must prove that the property owner intentionally or wantonly injured the plaintiff to recover. The exception is a child who is trespassing to play on ultra-hazardous items on the land. Since these trespassers are considered "anticipated" they are excepted under the doctrine of attractive nuisance.
A property owner may use reasonable (typically meaning nondeadly) force to prevent a person from trespassing on his, her or its land, or to expel a trespasser. However, a property owner may not force a trespasser off his land if doing so would expose the trespasser to a risk of serious injury. For example, a trespasser who takes shelter in a stranger's barn during a powerful storm cannot be expelled until the storm is over.
Many jurisdictions within the United States have passed statutes to modify or clarify the common law duties owed by a property owner to a trespasser (for example, by explicitly permitting the property owner to use deadly force to expel trespassers).