Current thinking in evolutionary biology holds that competition among individuals is the key to understanding natural selection. When competition exists, it is obvious that conflict arises; the emergence of cooperation, however, is less straightforward and calls for in-depth analysis. Much research is now focused on defining and expanding the evolutionary models of cooperation. Understanding the mechanisms of cooperation has relevance for fields other than biology. Anthropology, economics, mathematics, political science, primatology, and psychology are adopting the evolutionary approach and developing analogies based on it. Similarly, biologists use elements of economic game theory and analyze cooperation in "evolutionary games." Despite this, exchanges between researchers in these different disciplines have been limited. Seeking to fill this gap, the 90th Dahlem Workshop was convened. This book, which grew out of that meeting, addresses such topics as emotions in human cooperation, reciprocity, biological markets, cooperation and conflict in multicellularity, genomic and intercellular cooperation, the origins of human cooperation, and the cultural evolution of cooperation; the emphasis is on open questions and future research areas. The book makes a significant contribution to a growing process of interdisciplinary cross-fertilization on this issue.