The premature mechanical failure of engineering alloys in practice is often associated with exposure to service environments. Such localized phenomena are described as environmentally-induced embrittlement or environmentally assisted cracking, EAC. These phenomena have been known for a very long time. Materials engineers are fully able to make materials selection decisions for the construction of engineering systems that see service in virtually any environment, though circumstances may sometimes lead to unintended exposure that becomes problematic. What is not known with certainty at this stage is the mechanism by which the various modes of EAC occur. The goal in this presentation is to assess the current state of knowledge of EAC, to identify what is known in a mechanistic sense and what remains to be understood in terms of the path forward toward a more complete mechanistic understanding.