This book examines an important paradigm shift in biology: Plants and animals, traditionally viewed as individuals, are now considered to be complex systems and host to a plethora of microorganisms. After first presenting historical aspects of microbiota research, bacterial compositions of individual microbiomes and the critical analysis of current methods, the book discusses how microbial communities inside the human body are profoundly affected by numerous factors, such as macro- and micro-nutrients, physical exercise, antibiotics, gender and age. As described by current research, the author highlights how microbiomes contribute to the fitness of the host by providing nutrients, inhibiting pathogens, aiding in the storage of fat during pregnancy, and contributing to development and behavior. The author not only focusses on prokaryotic components in microbiomes, but also addresses single-cell eukaryotes and viruses.