Environmental stress failure of polymers (specifically glassy thermoplastics) is a major failure mode in application end use. The environmental stress failure discussion will primarily be limited to non-reactive environments. Crystalline polymers and even lightly crosslinked polymers can exhibit similar failure to glassy polymers but often require a much higher external applied stress. The mechanisms of environmental stress cracking proposed in the literature will be discussed in detail along with testing methods commonly employed. Approaches to resolve environmental stress failure have limited discussion in the open literature although a significant number of references can be found in the patent literature. Polymer blend approaches, fiberglass reinforcement, and impact modification are areas where industry has solved the environmental stress failure problems. An additional unique mode of failure with water exposure in glassy engineering polymers yielding microcavity formation will also be discussed. Environmental stress failure is an area of considerable concern to the utilization of polymers in a myriad of applications and the unsolved problems existing in the area will be detailed suggesting the need for additional research attention. POLYM. ENG. SCI., 2013. (c) 2012 Society of Plastics Engineers