Modeling plasticity in gold thin films on silicon substrates is discussed with particular reference to the evolution of stresses during thermal cycling. Following the work of Kobrinsky and Thompson, we show that plasticity of unpassivated Au films is dominated by constrained diffusional flow of matter from the free surface of the film to the grain boundaries. This diffusional effect depends strongly on film thickness, as expected. The presence of a 10 nm thick passivation layer of W inhibits these diffusional relaxation processes and causes the stress evolution in the film to be dominated by dislocation flow. It is shown that a simple kinematic strain hardening law for plasticity, suggested by Suresh, together with a temperature dependent elastic modulus, provides a good phenomenological account of the stress-temperature curves for passivated films. Using this modeling approach it is possible to draw a distinction between the yield strength of the film and the strain-dependent flow stress. Models of multiple misfit dislocation formation, developed separately by Willis and Freund, are used to rationalize the linear kinematic hardening laws used in this modeling. Efforts to model the stress-temperature behavior of unpassivated films by combining the effects of diffusional relaxation and dislocation mediated strain hardening will be reported.