Phenotypic plasticity allows a single genotype to produce multiple phenotypes in response to modifications of its environment, and many authors have proposed that it plays a significant role in determining evolutionary trajectories of populations. However, opponents of this view have focused on the ‘non-genetic’ nature of such phenotypic changes and counterargued that it is the genetic variation in plasticity that impacts evolutionary processes. This chapter emphasizes the environment-dependent nature of gene expression and highlights recent research that implicates plasticity per se as a major factor in evolutionary processes including the origin of novel phenotypes, speciation and population divergence, and the dynamics of evolutionary change. Some of the foundations of the broader controversy surrounding the importance of plasticity and other phenomena for evolutionary theory are examined. Finally, it is argued that the issues arising from consideration of phenotypic plasticity’s impacts reveal a lacuna in standard evolutionary theory: while providing a fairly comprehensive scheme for how existing variation is sorted, it lacks comparable components of a theory for how variation is produced.