The existence of living organisms in diverse ecosystems has been the focus of interest to human beings, primarily to obtain insights into the diversity and dynamics of the communities. This book discusses how the advent of novel molecular biology techniques, the latest being the next-generation sequencing technologies, helps to elucidate the identity of novel organisms, including those that are rare. The book highlights the fact that oceans, marine environments, rivers, mountains and the gut are ecosystems with great potential for obtaining bioactive molecules, which can be used in areas such as agriculture, food, medicine, water supplies and bioremediation. It then describes the latest research in metagenomics, a field that allows elucidation of the maximum biodiversity within an ecosystem, without the need to actually grow and culture the organisms. Further, it describes how human-associated microbes are directly responsible for our health and overall wellbeing.