This chapter extensively discusses the crystallization behavior of various types of polylactides (PLAs) with different molecular structures, configurations, and fillers in presence of dissolved CO2. These PLAs could be named as PLAs with different linear and branching structures, various D-lactide contents, and with micro-/nanosized additives. The effects of dissolved N-2 and helium are also compared with that of CO2. Slow crystallization kinetics of PLA is one of the main drawbacks, which result in poor foaming ability of PLA. Therefore, understanding the crystallization mechanisms and behavior of PLA in presence of dissolved blowing agent is of a great importance. Moreover, it will be disclosed how various dissolved blowing agents could influence the crystallization rate and kinetics of PLA and hence the foaming ability and subsequently the foam properties. High-pressure differential scanning calorimeter, X-ray diffraction, optical microscopies are a few techniques utilized to analyze the crystallization of PLA-gas mixtures.