Given that all species are confronted with a heterogeneous environment, why isn’t every species highly plastic? This chapter addresses one aspect of this question by exploring how natural selection can lead to the loss of phenotypic plasticity following a change in the environment, a process often referred to as genetic assimilation. Both quantitative genetic models and individual-based genic simulations suggest that the process of loss requires active selection against plasticity and can take thousands of generations. Yet, the few empirical studies that include data on all of the necessary components show that this loss can occur much more quickly. Laboratory studies also established that plasticity could evolve rapidly, but none have involved a step change in the environment; rather they involved a short-term stress, and so are not actual tests of genetic assimilation theory. While models, laboratory experiments, and data from natural populations all point to the plausibility of genetic assimilation contributing to evolution and highlight some of the conditions that favor genetic assimilation, more work is needed to understand this process.