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 Missouri, property owners can use 'No Trespassing' signs to deter unauthorized entry and reduce liability. Missouri law recognizes the importance of such signs to establish that individuals entering the property without permission are trespassing, which can lead to both civil and criminal consequences. Under Missouri statutes, a person commits the offense of trespassing if they enter unlawfully upon private property. This includes property that is clearly marked with 'No Trespassing' signs or purple paint (which is used to mark boundaries and indicate no trespassing areas). The presence of these signs or markings serves as a legal notice that entry is not allowed. Regarding liability, Missouri follows the general rule that landowners owe a lower duty of care to trespassers than to invitees or licensees. If a trespasser is injured on the property, the fact that the landowner had posted 'No Trespassing' signs may indeed strengthen their defense against liability claims, as it demonstrates that the trespasser was warned and entered the property at their own risk.