Our bodies are home to a vast sea of microorganisms. They reside inside us and on all our body surfaces. There are as many cells of these microbial partners as there are cells inside our bodies. The word microbiota describes all the organisms that are on our body surfaces as well as inside us. The important role of these partners of ours in our health and fitness has only been realized in the past ten years. They are invisible and do not receive the attention they deserve. The microbiota are a key component of our physical reserve and are vital to our health and fitness. The microbiota influence all of our organ systems, assist in digestion, disease resistance, contribute to metabolism, and are critical for the maintenance of health and fitness. A vital feature of the microbiota is their diversity of organisms—a wide variety of organisms are normally present. Our history with the microbiota is best described by the word coevolution - we evolved with them, and they evolved with us.The good news about the microbiota is that it is relatively easy to change bacterial populations in the gut through diet. Ways to do this are comprehensively outlined in the book.