The broad scope of evolutionary biology, operating across genes, organisms, clades and ecosystems, presents an ongoing challenge for those seeking to integrate the abundant new data and content available into a general theory of evolution going forward. This book presents seventeen chapters in which experts review the current state of research in evolution across disciplines, topics and constitutive theories, with a focus on integrating new findings with traditional approaches and views. The evolutionary topics range from biogeography, homology, development, life history, sex, natural selection, multi-level selection, phylogeny, tree of life studies, reticulate evolution and speciation to macroevolutionary theory, with overviews on recent growth in evolutionary theory’s content and on the history of evolutionary theory. This work is presented within the context of a more general theory of biology and is intended to build on a similar exploration in “The Theory of Ecology”, also from University of Chicago Press. The study of evolution is thriving as a basic science to understand the world, including its applications in medicine, public health and agriculture, and the book’s syntheses are intended to enhance future research and understanding.