Life on Earth depends on microbes to cycle key elements and acquire energy to drive food webs. Through their biochemical transformations, both helpful and harmful, diverse microbial activities largely determine the quality of soil, air, and water for human life. Now, for the first time in history, human technology rivals the ability of microbes to alter fundamental cycles of biogeochemistry. The burning of fossil fuels, which were generated millions of years ago by subterranean microbes, releases quantities of carbon dioxide too great to be absorbed by marine bacteria and algae. Growing rice production increases the release of methane by methanogens that thrive in submerged rice paddies. Both carbon dioxide and methane are “greenhouse gases” that lead to global warming. From local wastewater treatment to the control of greenhouse gases, microbes are our hidden partners on Earth. This chapter explains how microbes throughout the biosphere recycle carbon, nitrogen, sulfur, and other elements essential for all life. It also explores astrobiology, the study of whether biospheres exist on other worlds.