This chapter is your introduction to microbial diversity and to the many ways microbes shape the environment. Emphasis is placed on the ways microbes influence and interact with humanity and on human attempts to understand microbes. Because microbes are too small to be seen, people tend to underestimate their importance, but they profoundly affect human welfare. Microbial diseases have devastated populations, yet microbes have also led to important medical advances. For example, many antibiotics are microbial products, as are the tools of recombinant DNA technology. Recombinant human insulin to treat diabetics is produced in E. coli, and vaccines are developed from attenuated viruses. All the O2 gas in the atmosphere is ultimately present because certain bacteria evolved to carry out oxygenic photosynthesis. Your ability to gain energy using this oxygen via respiration in mitochondria is due to an ancient endosymbiosis that incorporated a prokaryote into a pre-eukaryotic cell. The bottom line is that humans would not be alive today were it not for the impacts microbes have had on our planet.