Environmental stress cracking is the formation of cracks in a material caused by relatively low tensile stress and environmental conditions. It describes one of the most common causes of unexpected brittle failure of thermoplastic (especially amorphous) polymers known at present. It shows the exposure of polymers to liquid chemicals tends to accelerate the crazing process, initiating crazes at stresses that are much lower than the stress causing crazing in air. It is the premature initiation of cracking and embrittlement of a plastic due to the simultaneous action of stress and strain and contact with specific fluids that are present in a gaseous state. It provides the key to the problem of predictability, failure is due to a combination of influences.