Understanding the factors that generate large-scale patterns in biotic diversity and community composition has been the work of ecologists for a century. Environmental gradients in temperature, precipitation, energy, and resources have been a useful template for understanding the ecological determinants of many biotic communities. We are currently entering an age in which we can uncover the composition and diversity of a great majority of soil and aquatic microorganisms, yet we currently lack a comprehensive understanding of the factors that structure the composition and diversity of the microbial world. Understanding the environmental determinants of microbial communities is a prerequisite to linking microorganisms to spatially explicit models of carbon and nutrient cycling, which are dominantly under their control. Environmental gradient analysis can been used to determine which factors structure microbial communities, and it can provide insight into how microbial communities respond to a changing world. The purpose of this symposium is to synthesize what is currently known about microbial communities across environmental gradients and the environmental factors that structure soil and aquatic microbial communities at small and large scales, to search for common themes, and to consider how microbial communities affect ecological processes. This symposium is also intended to synthesize interest from the two microbial-themed ESA sections, Soil Ecology and the newly formed Microbial Ecology section. A panel discussion will follow presentations.