Evolutionary transitions in individuality occur when the unit of selection and adaptation––the evolutionary individual––shifts from one level of organization to another. Examples include the origin of the genome from groups of cooperating genes, the origin of the eukaryotic cell from groups of prokaryotic cells, and the origin of multicellular organisms from unicells. This chapter examines how phenotypic plasticity may have contributed to evolutionary transitions in individuality by focusing on two keys steps in the transition from unicellular to multicellular life: group formation and cellular differentiation. In the volvocine green algae, a model system for studying the evolution of multicellularity, plasticity is present throughout these stages. Plastic cell-group formation is seen in the unicellular outgroup Chlamydomonas reinhardtii. The authors discuss tests of the hypothesis that plastic cell group formation was ancestral to the volvocine green algae by comparing environmentally induced cell groups, experimentally evolved cell groups, and colonial volvocine green algae. They then focus on cellular differentiation and hypothesize that plastic cellular differentiation may have been ancestral to a volvocine clade in which somatic cells have been gained and/or lost multiple times. Consistent with this hypothesis, undifferentiated Eudorina species develop cells that resemble soma following environmental stress; these cells are also observed in the offspring of colonies that have experienced environmental stress. The chapter concludes with a discussion of the role plasticity may have played in other evolutionary transitions in individuality, highlighting the evolution of multicellularity in animals and eusociality in insects. Overall, there is evidence that plasticity is present in key steps in evolutionary transitions in individuality.