Understanding and predicting how populations will react to changes in the environment is a long-standing goal in evolutionary ecology. It is also of considerable practical importance, as anthropogenic changes stress species worldwide. The relevance of phenotypic plasticity is becoming more apparent as species are forced to cope with rapid changes in the environment. This dissertation explores ways in which phenotypic plasticity will play a major role in determining the future of populations. In Chapters 1 and 2, I evaluate a modeling framework that could be used to predict plastic changes in key life history traits of ectotherms brought about by temperature. This work, based on the metabolic theory of ecology (MTE), assumes that biological rates scale exponentially with temperature. I first show the validity of the MTE for predicting lifespan gradients within species and then apply this temperature-life history relationship to predict changes in ectotherms resulting from global temperature increases over the next 50 years. In Chapter 3, I experimentally test the plastic response of sheepshead minnows, Cyprinodon variegatus, an estuarine fish common to the east coast, to combinations of temperature (24, 29, 34??C) and food availability (60, 80, or 100% of maximum consumption). The thermal response of juvenile growth rate was mediated by food availability, while the age at maturation was independently affected by temperature and food. Notably, and despite very different thermal and feeding regimes, the fish matured within a small size window. In Chapters 4 and 5, I explore transgenerational plasticity (TGP) as a means to cope with temperature changes. When the temperature experienced by the parents acts as a reliable indicator of thermal offspring environment, a parent can "pre-program" offspring traits appropriate for the predicted environment. This transfer of information from parent to offspring has been termed TGP, and is well studied in plants and invertebrates. In these chapters, I show that thermal TGP has a strong effect in larval growth of sheepshead minnows. I also explore how transgenerational and phenotypic plasticity interact to shape the size of fish throughout life, and provide evidence suggesting that the TGP effect lasts for at least 2 generations.