The resistance of organic coatings to environmental degradation is usually evaluated in terms of changes in physical properties or appearance. Thus, paint system weatherability is assessed in terms of embrittlement and cracking, loss of adhesion, and loss of gloss, while corrosion protection is assessed in terms of paint blistering, loss of paint adhesion, and pitting or perforation of the substrate. The design and evaluation of accelerated tests are based on correlation of the physical changes induced by the selected test exposure condition with those observed under some standard conditions (e.g., a defined course of natural exposure). Many of the changes in physical properties and appearance that result from environmental degradation have their origins in chemical degradation processes. Examination of the chemical origins of physical change affords a complementary approach and potentially an alternative to accelerated performance testing for the evaluation of protective coating systems. The underlying theme of chemical origin of paint performance suggests a chemical paradigm for study of paint degradation, which in turn leads to the development of chemical protocols for the evaluation of durability. The implications of the chemical paradigm for research methods and test design are discussed.