Book excerpt: Over the last two decades, research into epistasis has seen explosive growth and has moved the focus of research in evolutionary genetics from a traditional additive approach. We now know the effects of genes are rarely independent, and to reach a fuller understanding of the process of evolution we need to look at gene interactions as well as gene-environment interactions. This book is an overview of non-additive evolutionary genetics, integrating all work to date on all levels of evolutionary investigation of the importance of epistasis in the evolutionary process in general. It includes a historical perspective on this emerging field, in-depth discussion of terminology, discussions of the effects of epistasis at several different levels of biological organization and combinations of theoretical and experimental approaches to analysis.