By investigating a simple question, a philosopher of science and a molecular biologist offer an accessible understanding of microbial communities and a motivating theory for future research in community ecology.   Microorganisms, such as bacteria, are important determinants of health at the individual, ecosystem, and global levels. And yet many aspects of modern life, from the overuse of antibiotics to chemical spills and climate change, can have devastating, lasting impacts on the communities formed by microorganisms. Drawing on the latest scientific research and real-life examples such as attempts to reengineer these communities through microbial transplantation, the construction of synthetic communities of microorganisms, and the use of probiotics, this book explores how and why communities of microorganisms respond to disturbance, and what might lead to failure. It also unpacks related and interwoven philosophical questions: What is an organism? Can a community evolve by natural selection? How can we make sense of function and purpose in the natural world? How should we think about regeneration as a phenomenon that occurs at multiple biological scales? Provocative and nuanced, this primer offers an accessible conceptual and theoretical understanding of regeneration and evolution at the community level that will be essential across disciplines including philosophy of biology, conservation biology, microbiomics, medicine, evolutionary biology, and ecology.