The concept of fitness has long been a topic of intense debate among evolutionary biologists and their critics, with its definition and explanatory power coming under attack. In this book, Richard Michod offers a fresh, dynamical interpretation of evolution and fitness concepts. He argues that evolution has no enduring products; what matters is the process of genetic change. Whereas many biologists have focused on competition and aggression as determining factors in survival, Michod, by concentrating on the emergence of individuality at new and more complex levels, finds that cooperation plays even a greater role. Michod first considers the principles behind the hierarchically nested levels of organization that constitute life: genes, chromosomes, genomes, cells, multicellular organisms, and societies. By examining the evolutionary transitions from the molecular level up to the whole organism, the author explains how cooperation and conflict in a multilevel setting leads to new levels of fitness. He builds a model of fitness drawing on recent developments in ecology and multilevel selection theory and on new explanations of the origin of life. Michod concludes with a discussion of the philosophical implications of his theory of fitness, a theory that addresses the most fundamental and unique concept in all of biology.