The “balance of nature” or “nature´s equilibrium” is an idea discussed in scientific ecology but also in popular writing. Especially in times of environmental crisis it has gained much influence. But what is behind this concept? This book investigates the history and epistemology of “equilibrium” in population ecology where it is a contentious issue. Asking “why do population ecologists use the concept?” this book describes the dynamics of the idea of “balance” from the 1930s to its present use. It is inspired by approaches used in science studies, putting emphasis on the viewpoints of scientists. Its interdisciplinary character makes it relevant reading for population biologists, historians and philosophers of science.