The environment includes an almost infinite variety of microorganisms and their products. The human microbiome is influenced by this environmental microbiome and its interconnectedness, which recognizes these environmental elements as part of a complex ecosystem impacting human health. The recognition that humans co-evolved with microbes has contributed to the acknowledgment that different ecological communities of commensal, symbiotic, and pathogenic microorganisms share and contribute to our body. Microbiomes include species across all major kingdoms, including viruses as well as archaea, bacteria, and microbial eukaryotes. Recent research is rapidly strengthening the appreciation that associated microorganisms make essential contributions to human health and environmental well-being. This volume is an exploration of the environmental microbiome.