THIS CHAPTER REVIEWS AND UPDATES THE TOPICS addressed by the previous authors, including up-to-date references and expanded to include stress in multi-layer systems and stress in physical aging. Interest in the stress phenomena in organic coatings is relatively new. The importance of understanding and measuring this stress became evident as facts accumulated about its role in coating degradation [1–10]. This is also supported by the fact that many coatings used today (e.g., thermosets) are more susceptible to developing high stresses than traditional ones (e.g., alkyd paints). It is now quite clear that stress can affect coating adhesion and/or cohesion and provoke delamination and/or cracking. Since the development of stress is involved in practically all stages of coating life (film formation, exposure to various climatic conditions), its measurement is essential for a better understanding of coating behavior.