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 Washington State, trespassing is defined as the unauthorized entry onto someone else's land or property without permission. This can include not only a person physically entering the property but also causing an object to enter the property. The property owner has the right to exclude others, except in certain situations such as easements. To establish a trespass claim in Washington, the plaintiff must demonstrate ownership or the right to possess the property, show that the defendant intentionally and voluntarily entered the property, and prove that the trespass resulted in harm to the plaintiff's possessory rights. It is important to note that the defendant's knowledge or ignorance of the property's ownership is not relevant; the key factor is the intentional act of entering the property. If these elements are proven, the plaintiff may be entitled to recover damages for the trespass.