‘Phenotypic plasticity,’ the expression of different phenotypes by a given genotype under different environmental conditions, is commonly observed in biochemical, physiological, behavioral, and morphological traits. The term includes both adaptive and nonadaptive reactions to environments. This chapter provides a historical survey of how plasticity has figured in evolutionary thought and research on how plasticity has been described; on plasticity as an adaptation with benefits, costs, and limits; on evolutionary compensation for maladaptive plasticity; on plasticity as a mechanism that may delay or avert extinction; and especially on the related ideas of genetic assimilation, genetic accommodation, and ‘plasticity-led’ evolution of traits. Although these last topics have been subject to debate, they are compatible with the theory that emerged from the Evolutionary Synthesis. The ability of plasticity to rescue populations in changing environments and many aspects of plasticity-led evolution including its prevalence, its basis in adaptive or nonadaptive plasticity, and its developmental, genetic and perhaps epigenetic bases are all questions for ongoing and future research.