Trespass is the unauthorized entry upon the land (real property) of another. Trespass may occur when one enters—or causes something to enter—another's property without permission. The law has traditionally recognized a claim or cause of action for damages based on trespass.

The owner of real property generally has the right to exclude all others from use of the property—although this right to exclude others may be relinquished in limited circumstances, such as when there is an easement allowing another to use the property. As a general rule, any unauthorized entry upon land of another is a trespass even when there is no damage or only slight damage.

Thus, to prove a trespass claim, the plaintiff must show that it owned the property or had a right to exclude others from the property. Some courts have recognized that the right to explore for oil and minerals is a valuable property right that can be legally protected through a trespass claim.

To recover damages for trespass to real property, a plaintiff must prove that: (1) the plaintiff owned or had a lawful right to possess real property; (2) the defendant entered the plaintiff’s land and the entry was physical, intentional, and voluntary; and (3) the defendant’s trespass caused injury to the plaintiff’s right of possession.

The only relevant intent is that of the actor (defendant) to enter the property. The actor’s subjective intent or awareness of the property’s ownership is irrelevant.

In New Hampshire, trespass refers to the unauthorized entry onto another person's land without permission. The law in New Hampshire recognizes trespass as a civil wrong, and a property owner can bring a claim for damages if someone unlawfully enters their land. To establish a trespass claim in NH, the plaintiff must demonstrate ownership or the right to possess the property, show that the defendant intentionally and physically entered the land, and prove that the trespass caused harm to the plaintiff's possessory rights. It is important to note that the defendant's intent to trespass is not judged by their knowledge of the property's ownership but rather by their intentional act of entering the property. While the law generally protects the right of property owners to exclude others, there are exceptions, such as easements, which may permit entry under specific circumstances. An attorney can provide more detailed advice on trespass laws and property rights in New Hampshire.