No trespassing signs can be useful to property owners—especially on large tracts of land or land that is not developed—to put persons entering the property on notice that the property is private property and if they do not have permission to enter, they may be committing civil and criminal offenses related to trespassing. In some states notice is required for a trespasser to have civil or criminal liability for entering the property without permission.

No trespassing signs may also protect a landowner against liability to a trespasser, as a lower duty of care is owed to trespassers in some states, and putting a person entering the property on notice that they are trespassing may strengthen the property owner’s defense against any liability claims if the trespasser is injured on the property.

In Pennsylvania, property owners can use 'No Trespassing' signs to deter unauthorized entry and reduce liability. Under Pennsylvania law, specifically Title 18 Pa.C.S.A. Section 3503, trespassing is a criminal offense when a person enters or remains on a property where such signs are posted without permission. The signs must be conspicuous and designed to notify potential trespassers that they are entering private property. The presence of these signs can also impact the duty of care a landowner owes to individuals on their property. Generally, a landowner owes a lower duty of care to a trespasser than to an invitee or licensee. By posting 'No Trespassing' signs, a landowner may strengthen their defense against liability claims if a trespasser is injured on the property, as it helps to establish that the trespasser was on the property without consent and had been warned against entering. However, this does not absolve a landowner from liability for willful or wanton misconduct that causes harm to a trespasser.