The previous chapters primarily discussed the mechanical behavior of materials under external mechanical loads. But, in fact, materials and components are always used in certain ambient media. Environmental media refers to the media that surrounds the materials (including gas, liquid, and solid media) and allows certain interface reactions with the materials. For example, bridges and steel rails are surrounded by a damp atmosphere; oil platforms and vessel hulls are surrounded by sea water; underground oil and gas pipelines, and cables, are surrounded by soil. Synergic action and mutual promotion between the environmental media and stress will cause faster material damage, crack initiation, and growth. Therefore, material damage under the combined action of environmental media and stress is more severe than under the independent action or simple summation of the two factors.