Superplasticity refers to the ability of fine-grained materials (d < 10 m, where d is the grain size) to exhibit extensive neck-free elongations during deformation at elevated temperatures (T > 0.5 T-m, where T-m is the melting point). An important characteristic of the deformation behavior of micrograin superplastic alloys is the experimental observation that the relationship between stress and steady-state creep rare is often sigmoidal. Such sigmoidal behavior is characterized by the presence of three regions: region I (the low-stress region), region II (the intermediate-stress region or the superplastic region), and region III (the high-stress region). Recently, the effects of impurity level and type on the sigmoidal relationship reported for superplastic alloys have been studied in detail. Some of the results of these studies are reviewed with particular emphasis on creep behavior, boundary sliding, and cavitation.