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 Colorado, trespassing is defined as the unauthorized entry onto another person's land without permission. The law in Colorado recognizes trespass as a civil wrong (a tort) and, in some cases, a criminal offense. To establish a civil trespass claim in Colorado, the plaintiff must demonstrate that they owned the property or had the right to exclude others, that the defendant made an intentional and voluntary physical entry onto the property, and that this entry caused harm to the plaintiff's possessory rights. It is not necessary for the plaintiff to prove that the defendant was aware that the property belonged to someone else; the defendant's intent to enter the property is sufficient for a trespass claim. Additionally, Colorado law protects the rights of property owners to explore and extract natural resources, such as oil and minerals, and unauthorized interference with these rights can also constitute trespass. Criminal trespass in Colorado is categorized into first, second, and third degree, with varying levels of penalties, depending on factors such as whether the trespasser entered a dwelling or if the entry was made with the intent to commit a crime.