Plastics may be subjected to mild stress and strain without failure or they may be subjected to weak solvents or aqueous surfactants without failure, but when they are subjected to combination of these two types of variables they often suffer catastrophic brittle fracture. Such failure has been named environmental stress-cracking (ESC). Environmental stress-crack resistance (ESCR) is generally directly related to molecular weight, narrow molecular weight distribution, and low crystallinity. Frozen-in stresses from processing aggravate ESC and are relieved by annealing, but further annealing also promotes crystallization which aggravates ESC. Polyaxial stress is often mentioned as an essential factor in ESC, and strain-induced orientation may be important in the mechanism of cracking.