A trespasser is someone who has violated the space or invaded the thing of another in a way that is contrary to the governing law. The law of trespass comes under the common law of torts, and in some circumstances can also be prosecuted as a crime. There are typically three torts of trespass: trespass to land; trespass to chattels, or things; and trespass of the person. A trespasser is the person accused of committing the trespass.
The contours of tort law and what precisely defines the tort of trespass varies by jurisdiction, but the general theme is that one’s space and things hold a certain integrity that cannot be impinged upon without permission. Trespass to land is perhaps the most commonly considered trespass. Signs declaring “No Trespassing” are popular among property owners. In most cases, however, signs and warnings are not necessary to define trespass: if a person crosses into land that is not his, by law, he is usually a trespasser.
Nevertheless, merely crossing through land is not typically a trespass that the law will punish. Under the earliest common law system, developed in Medieval England, setting foot on another’s land, or crossing through an estate which was not one’s own, was typically punishable. That is not usually the case anymore. Most countries with tort of trespass laws only penalize trespassers who have caused some definable harm, have acted with demonstrable negligence, or have violated a discreet right.
The tort of trespass to chattels is similar, only involving physical property in place land. In law, “chattels” is a word synonymous with “things.” Most trespass to chattels cases involve the wholesale unauthorized use or destruction of another’s goods. Those goods can be both tangible, like a car, or intangible, like the storage space on an e-mail server. A person who interferes with or damages another’s goods, or a person who prevents a property owner from making full use of his things, is generally a trespasser.
The third category of trespass, trespass on the person, involves one person’s undue interference with the sanctity of another’s personal space. Assault, battery, and false imprisonment are the classic physical trespasses. Typically, to be considered a trespasser of another’s person, one must act intentionally.
Depending on the facts, a trespasser’s transgressions might also rise to the level of criminal violations. The assault of another can always come under the law of tort, but if the assault is egregious enough — a premeditated attack, for instance, or a gruesome beating — it may be punishable under a criminal law code as well. This duality applies to all three categories of torts. Arson is often both a criminal trespass to land and a tort, as is burglary. Unauthorized bank account access can similarly be both a trespass to chattels and a criminal trespass when large-scale financial fraud is implicated.
Whether it comes under the civil or the criminal law, trespass is always defined by the harm it causes. Trespass laws exist in many respects to provide redress for people who have been harmed by another’s invasion. There are certain exceptions, however, and as with all legal claims, there are always defenses.
If an accused trespasser can show that his alleged trespass was permitted or consented to by the plaintiff, he can typically escape the charges. Necessity is also a defense in many places. Self defense is a very common defense to trespasses of the person, and sometimes also of land. When that land comprises one's home, the law often favors the homeowner.
Most jurisdictions recognize the sanctity of one’s home, and often extend special rights of self and family protection against unwanted intruders. In most normal circumstances, if one is approached and threatened by another, one has a general duty to retreat — or at least try to retreat — before retaliating. A trespass into a home eliminates that duty in some places, known as “castle doctrine” jurisdictions. The castle doctrine stipulates that someone threatened with attack in his own home can use force, including deadly force, to stop the attack without retreating first. A trespasser injured by a homeowner in this circumstance would have no recourse.