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 Arizona, trespassing is defined as knowingly entering or remaining unlawfully on any real property after a reasonable request to leave by the owner or any other person having lawful control over such property. This includes situations where someone enters or causes something to enter another's property without permission. To establish a trespass claim in Arizona, the plaintiff must demonstrate ownership or the right to possess the property, show that the defendant intentionally and physically entered the property, and prove that the trespass caused harm to the plaintiff's possessory rights. The defendant's intent to enter the property is the key factor, not their knowledge or awareness of the property's ownership. Trespass can lead to both civil liability for damages and criminal penalties. Arizona law provides for different classifications of criminal trespass, which can range from a class 3 misdemeanor to a class 6 felony, depending on the circumstances of the trespass.