This book provides a fundamental understanding of the importance of the microbiome in the life of virtually all multicellular organisms. It explains why microbes are an integral part of living organisms and describes the diverse roles they perform for their hosts. Although the significance of modified bacteria such as the mitochondrion and chloroplast is deeply rooted in the evolution of all complex organisms, it is only recently that the contribution of microbial partners within and on their hosts is becoming fully evident. These communities of microbes are as essential to organisms as are the visible organs. Microbiomes are indeed “invisible organs.” They participate in the digestive process, assist in communication networks, supply essential nutrients, guard against foreign intrusion, promote development and contribute to well-being. This unique approach, where the dependence of the hosts on their microbiomes is explained, will be a must-read for undergraduate and graduate students. Researchers and professionals probing microbial interactions with living organisms will find this book interesting and enriching.