This book is a must-read for every biologist interested in evolutionary biology. Schlichting and Pigiucci provide a clear and accessible overview of the current state of research on reaction norms in phenotypic evolution. The book's main goal is to argue that phenotypic plasticity should be considered an integral part of the evolutionary process, not just a peripheral phenomenon. The authors present a historical perspective, an assessment of current thinking, and their own views on future research directions. The first two chapters offer an overview of approaches and a historical review of the development of ideas. These chapters are particularly enjoyable and show how some evolutionary biologists, such as Waddington, were marginalized during the neo-Darwinian synthesis but have recently been rehabilitated. Chapters 3 to 9 analyze phenotypic evolution using the reaction norm perspective, while chapter 10 provides an overall summary and 14 potential research projects, which should be of particular value for graduate students in this field. While the authors attempt to cover the entire field of phenotypic evolution, they occasionally include areas that do not fit well into their perspective. For example, their chapter on allometry is interesting but in their last section, they assert that correlation coefficients are themselves allometric coefficients, which stretches the definition of allometry. However, the section is still informative and should be viewed as the evolution of suites of correlations. The book is provocative and will likely promote debate, which is a sign of success. One example of where the book provoked me is Schlichting and Pigiucci's view of quantitative genetic analysis as a temporary necessary evil that will be eliminated once we have better mechanistic models. They present a model that supposedly demonstrates that different phenotypic outcomes are possible even with identical correlations and phenotypic optima. However, the model is a single locus, two-allele model, which is not representative of quantitative genetics. The authors' model does show that it is possible to construct models with aberrant behavior, but it does not demonstrate that this behavior is typical of quantitative genetic models. Despite some disagreements, the book has enormous value. It has brought together very disparate sets of data under a common umbrella and thus provides a unifying theme in evolutionary biology, which is surely one of our major goals.This book is a must-read for every biologist interested in evolutionary biology. Schlichting and Pigiucci provide a clear and accessible overview of the current state of research on reaction norms in phenotypic evolution. The book's main goal is to argue that phenotypic plasticity should be considered an integral part of the evolutionary process, not just a peripheral phenomenon. The authors present a historical perspective, an assessment of current thinking, and their own views on future research directions. The first two chapters offer an overview of approaches and a historical review of the development of ideas. These chapters are particularly enjoyable and show how some evolutionary biologists, such as Waddington, were marginalized during the neo-Darwinian synthesis but have recently been rehabilitated. Chapters 3 to 9 analyze phenotypic evolution using the reaction norm perspective, while chapter 10 provides an overall summary and 14 potential research projects, which should be of particular value for graduate students in this field. While the authors attempt to cover the entire field of phenotypic evolution, they occasionally include areas that do not fit well into their perspective. For example, their chapter on allometry is interesting but in their last section, they assert that correlation coefficients are themselves allometric coefficients, which stretches the definition of allometry. However, the section is still informative and should be viewed as the evolution of suites of correlations. The book is provocative and will likely promote debate, which is a sign of success. One example of where the book provoked me is Schlichting and Pigiucci's view of quantitative genetic analysis as a temporary necessary evil that will be eliminated once we have better mechanistic models. They present a model that supposedly demonstrates that different phenotypic outcomes are possible even with identical correlations and phenotypic optima. However, the model is a single locus, two-allele model, which is not representative of quantitative genetics. The authors' model does show that it is possible to construct models with aberrant behavior, but it does not demonstrate that this behavior is typical of quantitative genetic models. Despite some disagreements, the book has enormous value. It has brought together very disparate sets of data under a common umbrella and thus provides a unifying theme in evolutionary biology, which is surely one of our major goals.