Trespass to land is a common law tort or crime that is committed when an individual or the object of an individual intentionally (or, in Australia, negligently) enters the land of another without a lawful excuse. Trespass to land is actionable per se. Thus, the party whose land is entered upon may sue even if no actual harm is done. In some jurisdictions, this rule may also apply to entry upon public land having restricted access. A court may order payment of damages or an injunction to remedy the tort.

By law, trespass for mesne profits is a suit against someone who has been ejected from property that did not belong to them. The suit is for recovery of damages the trespasser caused to the property and for any profits he or she may have made while in possession of that property.

For a trespass to be actionable, the tortfeasor must voluntarily go to a specific location, but need not be aware that he entered the property of a particular person. If A forces B unwillingly onto C's land, C will not have action in trespass against B, because B's actions were involuntary. C may instead claim against A. Furthermore, if B is deceived by A as to the ownership or boundaries of C's land, A may be jointly liable with B for B's trespass.

In most jurisdictions, if a person were to accidentally enter onto private property, there would be no trespass, because the person did not intend any violation. However, in Australia, negligence may substitute the requirement for intent.

If a trespass is actionable and no action is taken within reasonable or prescribed time limits, the land owner may forever lose the right to seek a remedy, and may even forfeit certain property rights. See Adverse possession and Easement by prescription.

Trespass may also arise upon the easement of one person upon the land of another. For example, if A grants B a right to pass freely across A's land, then A would trespass upon B's easement by erecting a locked gate or otherwise blocking B's rightful access.

In some jurisdictions trespass while in possession of a firearm, which may include a low-power air weapon without ammunition, constitutes a more grave crime of armed trespass.

The maxim "cuius est solum, eius est usque ad coelum et ad infernos" (whoever owns the land owns it all the way to heaven and to hell) is said to apply, however that has been limited by practical considerations. For example, aerial trespass is limited to airspace which might be used (therefore aeroplanes cannot be sued). Landowners may not put up structures to prevent this. The courts have been more lenient with putting up structures to prevent underground trespass. The Kentucky Court of Appeal in Edwards v Sims (1929) 24 SW 2d 619 seems to affirm the maxim without qualification, whereas the New South Wales Supreme Court in Australia seemed more reluctant to do so in Di Napoli v New Beach Apartments (2004) Aust Torts Reports 81-728. There is therefore an asymmetry between aerial and underground trespass, which may be resolved by the fact the ground is almost always used (to support buildings and other structures) whereas airspace loses its practical use above the height of skyscrapers.

There may be regulations that hold a trespasser to a higher duty of care, such as strict liability for timber trespass (removing trees beyond a permitted boundary), which is a type of trespass to chattels as a result of a trespass to land.

Some cases also provide remedies for trespass not amounting to personal presence, as where an object is intentionally deposited, or farm animals are permitted to wander upon the land of another. Furthermore, if a new use of nearby land interferes with a land owner's quiet enjoyment of his rights, there may be an action for nuisance, as where a disagreeable aroma or noise from A drifts across the land of B.

Trespass ab initio is when a person is granted access to land but then abuses that access. The entry to the land is considered to have been a trespass from the beginning. This only applies to access given by law, not to access given by a person (as established by the Six Carpenters' Case).