The microbiome is an assemblage of a complex community of microbes (bacteria, archaea, fungi, algae, protists, and viruses) and their biomolecules occupying a well-defined habitat in or on a living or non-living object. All the environmentally exposed surfaces of the human body are colonized with trillions of microbes from all three major domains of life, including bacteria, archea, and microscopic eukarya. However, the richness, abundance, and functional potency of microbial taxa living in different parts of the human body are distinct. The Presence of common microbial taxa in different body habitats is also very rare. With the recent development of next generation sequencing technologies, it has been established that the indigenous microbial community in the human body and their functional attributes within a given body habitat vary over time, between ethnic groups and health status of the host. Perturbation of homeostasis in community structures or functions due to any extrinsic factors can alter mutualistic host-microbe interactions and may lead to disease. In addition, the dysbiotic state of the microbiome can also affect the efficacy of therapeutics, prolong treatment duration and lead to undesired treatment outcomes. In this chapter, structure, functions, diversity and dynamics of human microbiome in health and diseases, factors that alter microbial composition, interactions between microbial taxa and xenobiotics, and therapeutic efficacy of drugs in dysbiotic conditions are highlighted.