The polymers commonly used in plastic electronic packagingare apt to absorb moisture in the environment, which greatly affects thereliability of packages. In this article the studies on moisture diffusion process,hygroscopic stresses and moisture-induced vapor pressure arereviewed, with the emphasis on the theoretical analysis,parameter characterization and its experimental measurement and validation,as well as the finite element simulation. Based on the mechanism analysisand experimental results, it can be seen that moisture absorption of polymermaterials in plastic packages can degrade its reliability significantly.More and more researches are focused on the study of moisture absorption,diffusion and evaporation processes, and on the experimentalmethods to measure and characterize the above processes.