Liability for injury due to hazardous products often hinges on the safety of the defendants product relative to the safety of similar products. For instance, firms that can show their product's safety was "state of the art" can in some cases have their liability removed. This paper explores the legal definition of what it means to be state of the art and considers whether or not the availability of the defense is likely to improve product safety. The state of the art defense's effect on safety is found to depend on whether courts rely on a "technological advancement" or a "customary practice" tests of state of the art. When consumers are under-informed regarding product risks, the technological advancement test improves safety, and welfare, in a broad set of situations.