Motivated by demanding applications of polyimides and polyimide matrix composites, this study aims to understand the mechanical behavior of HFPE-II-52 polyimide at high temperature. First, a temperature dependent constitutive model combining linear viscoelasticity with viscoplasticity was developed. The viscoplastic part of the model uses a power law flow potential with state variable evolution. The full model was fit to a set of tension tests including constant strain rate, multistep stress relaxation, and creep and recovery tests in a range of temperature 285-315C. Second, the effects of moisture on the mechanical properties of polyimide were investigated. Separate experiments were designed to study the effects of both hydrolytic degradation and plasticization. The experiments consist of exposing the material sample to high temperature, moisture saturated conditions over a range of times and temperatures. Following moisture exposure, compression tests were performed to measure the reductions of stiffness and yield stress. A temperature and moisture dependent kinetic model was then developed and was integrated with the previous viscoelastic and viscoplastic model. Third, under certain hygrothermal conditions such as rapid heating with moisture saturated polyimide, the material may fail by high pressure water vapor induced blistering. Built on prior modeling efforts, a finite element approach is used to simulate the material unstable void growth. The simulation approach provides a means for the prediction of the critical temperature of blistering under different heating rates and moisture levels and allows for an investigation of the importance of the effects of pressure, thermal softening, hydrolytic degradation and plasticization on the blistering failure of polyimide.