This book provides an interdisciplinary exploration of biological individuality, by identifying leading and less familiar perceptions of individuality, what they are good for, and in what contexts. Individuals are things that everybody knows—or thinks they do. Yet even scholars who practice or analyze the biological sciences don't agree on what an individual is and why. One reason is that biological individuality concepts serve different purposes—defining, classifying, or explaining living structure, function, interaction, persistence, or evolution. Practice and theory point to individuals at different levels of organization, from genes to cells to organisms to symbiotic systems. Notions of individuality engage theoretical questions about multilevel natural selection and empirical questions about development, function, and ecology. They ground philosophical questions about the nature of living things and causation. They reflect particular historical and cultural circumstances, often in interaction with theorizing about the nature of society. Each approach has been contested, demonstrating the difficulties of reconciling concepts focusing on structures and those oriented toward processes. Ten scholarly essays and three commentaries by interdisciplinary biologists, historians, and philosophers detail many of the ideas, major figures, and empirical studies both past and present that show why there is no simple answer to the question, "What is an individual, biologically speaking?" There isn't and shouldn't be a single answer. Nature is too messy for that, organisms too quirky in the diverse ways they reproduce, function, and interact, and human ideas about individuality too fraught with philosophical and historical meaning.