There is a long history of studying stress in thin films. A wide range of phenomenology has been observed showing a complex dependence on growth rate, temperature, microstructural evolution and particle energy. The experiments bring up interesting questions about the underlying kinetic processes that determine stress. For instance, why does it change form tensile to compressive in many metal films as they get thicker? Or why does a film grow with tensile stress at low temperatures but with compressive stress at higher temperatures? To address observations such as these, we describe a model that considers multiple mechanisms for generating and relaxing stress during film growth. These relate the stress to fundamental materials processes such as grain boundary formation, adatom diffusion, grain growth and defect formation. The resulting equations describe the stress evolution as a function of thickness so that the model can be directly compared with real-time wafer curvature measurements. Examples for numerous materials, deposition processes and processing conditions will show how we are beginning to develop a comprehensive understanding of thin film stress that will allow us to control and predict it.