Locusts exhibit extreme density-dependent phenotypic plasticity, in which cryptic, solitary individuals transform into conspicuous, gregarious individuals in response to change in population density. In addition to color and behavior, morphology, endocrine action, biochemistry, nutritional intake, and genetic expression also change, and this reversible density-dependent transformation is commonly referred to as locust phase polyphenism. Recent studies have advanced our understanding of the processes behind behavioral and physiological phase transformation, but how locust phase evolved has remained only speculative. Here I attempt to understand the evolution of locust phase polyphenism from phylogenetic perspective. Phylogenetic analysis of Cyrtacanthacridinae suggests that locust phase polyphenism evolved multiple times within the lineage. It also suggests that locust species are often closely related to non-locust species which exhibit certain forms of density-dependent polyphenism. Because locust phase polyphenism is highly complex, it can be considered a composite character that can be broken into many smaller components. When these behavioral, ecological, and physiological components of locust phase are optimized on to the phylogeny, it becomes evident that certain components are phylogenetically conserved. Specifically, physiological plasticity appears to be more conserved than behavioral plasticity. Therefore, it is important to understand both pattern and process to study the evolution of locust phase polyphenism.