Of all organisms, microbes grow in the widest range of habitats, from Antarctic lakes to the acid drainage of iron mines, as well as the air, water, and soil that surround us. Wherever found, microbes both respond to and modify the environment that surrounds them. Microbial communities form the foundation of Earth’s biosphere, shaping the environments inhabited by plants and animals. In the ocean, vast quantities of microbes produce the biomass that ultimately feeds fish and humans. In forests and fields, microbes are actually the main consumers; they decompose the majority of plant material, generating fertile soil. Through their biochemistry, diverse microbes largely determine the quality of soil, air, and water for human life. This chapter explores how microbes interact with each other and with their many diverse habitats on Earth.