The level of security afforded by tonneau covers is dependent on the material used in its construction as well as its locking mechanisms. Vinyl or fabric materials can be punctured or slashed with a sharp object. Hardtops and retractable tonneau cover units, on the other hand, make it a bit tougher to crack. Some even have integrated lock systems at the tailgate to improve security. Regardless, simply covering the contents of your truck bed should reduce the risk of theft.