We are at the dawn of a new scientific revolution. Our understanding of how to treat and prevent diseases has been transformed by knowledge of the microbiome-the rich ecosystem of microorganisms in and on every human. These microbial hitchhikers may hold the keys to human health. In Gut Feelings, Alessio Fasano and Susie Flaherty show why we must go beyond the older, myopic view of microorganisms as our enemies to a broader understanding of the microbiome as a parallel civilization that we need to understand, respect, and engage with for the benefit of our own health. Recent advances in understanding the microbiome and its role in human health dovetail with the development of personalized or "precision" medicine to create treatments and prevention programs targeted to the molecular imprint of an individual. Fasano and Flaherty explore the microbiome's part in such diseases as gut inflammatory disorders, obesity, neurological conditions, and cancer, and they explain new research in prebiotics, probiotics, synbiotics, and psychobiotics. They also discuss the microbiome and immune function, including a possible role in COVID-19 treatment. By simultaneously expanding our perspective to encompass large datasets and multiple factors in human health, and narrowing our focus to identify the individual communities in the human microbiome, we will enlarge-and perhaps reinvent-our understanding of how to combat disease and maintain health. Why the microbiome--our rich inner ecosystem of microorganisms--may hold the keys to human health. We are at the dawn of a new scientific revolution. Our understanding of how to treat and prevent diseases has been transformed by knowledge of the microbiome-the rich ecosystem of microorganisms in and on every human. These microbial hitchhikers may hold the keys to human health. In Gut Feelings, Alessio Fasano and Susie Flaherty show why we must go beyond the older, myopic view of microorganisms as our enemies to a broader understanding of the microbiome as a parallel civilization that we need to understand, respect, and engage with for the benefit of our own health. Recent advances in understanding the microbiome and its role in human health dovetail with the development of personalized or "precision" medicine to create treatments and prevention programs targeted to the molecular imprint of an individual. Fasano and Flaherty explore the microbiome's part in such diseases as gut inflammatory disorders, obesity, neurological conditions, and cancer, and they explain new research in prebiotics, probiotics, synbiotics, and psychobiotics. They also discuss the microbiome and immune function, including a possible role in COVID-19 treatment. By simultaneously expanding our perspective to encompass large datasets and multiple factors in human health, and narrowing our focus to identify the individual communities in the human microbiome, we will enlarge-and perhaps reinvent-our understanding of how to combat disease and maintain health.