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 Wisconsin, property owners can use 'no trespassing' signs to deter unauthorized entry and reduce liability. Under Wisconsin Statutes section 943.13, trespassing is prohibited on lands where signs are posted at intervals of not more than 1,000 feet and at all road, trail, and waterway entrances, or in a manner reasonably likely to come to the attention of intruders. The signs must be at least 11 inches square and carry a clear 'no trespassing' message. The presence of these signs can establish that a person entering the property without permission is aware they are trespassing, which is necessary for criminal liability. Additionally, Wisconsin follows the common law tradition that landowners owe a lower duty of care to trespassers than to invitees or licensees. Posting 'no trespassing' signs can help landowners argue that they have fulfilled their limited duty to trespassers, potentially strengthening their defense against liability claims if a trespasser is injured on the property.