This chapter provides a basis for consideration of mechanical properties, based on a microscopic picture that has developed from experimental observation. In many cases, mechanical properties are the most important factor in determining potential applications of a material. Some high-temperature superconductors have very useful electrical and magnetic properties but are limited in their applications due to their mechanical properties. Even metals can become amorphous or glassy when they are cooled so rapidly that the atoms cannot arrange themselves in the more stable crystalline form. Spider silk has amazing mechanical properties, and we now understand these properties at a molecular level. The microstructure, that is structure at the sub-mm length scale, can play a very important role in mechanical properties of materials. One important factor concerning the mechanical strength of a material is the response of its structure to stress. Many polymers change their mechanical properties as a function of temperature.