Microcellular semicrystalline polymers such as poly(ethylene terephthalate) show great promise for engineering applications because of their unique properties, particularly at higher densities. Recent studies have shown that some high density microcellular polymers have longer fatigue lives and/or equivalent strengths relative to the neat polymer. Relatively few microcellular processing studies of semicrystalline polymers have been presented. In general, semicrystalline polymers are relatively difficult to microcellular process compared to amorphous polymers. In this paper, the microcellular processing of poly(ethylene terephthalate) in the amorphous and semicrystalline states is studied in order to quantify the processing differences. Particular emphasis is given to the cell growth stage of microcellular processing comparing the processing characteristics of semicrystalline and amorphous PET. Based on the results of this study, a number of critical process parameters are identified. Another goal of this analysis is to derive robust microcellular process design strategies for the amorphous and semicrystalline materials. Process design strategies are given for both semicrystalline and amorphous PET.