Solid waste landfills are a ubiquitous final repository for heterogeneous mixtures of residential, commercial and industrial waste. Microorganisms drive the decomposition of the solid waste in a highly structured environment that is physically and chemically heterogeneous. In doing so, the landfill quickly becomes anaerobic and limited for electron acceptors. Decomposition also solubilizes a wide array of organic and inorganic compounds, producing leachate and “landfill gas” (mainly CO2, H2 and CH4) that can have a profound effect on groundwater and the atmosphere. Solid waste landfills represent a novel selective landscape likely to have a profound effect on the microbial inhabitants, which have not been characterized extensively. A national survey of landfill leachate conducted in collaboration with the USGS provided the opportunity to characterize the microbial assemblages found in 19 non-hazardous landfills across 16 states in the U.S. These landfills varied in size, age of waste, waste composition, and management strategy. The landfills were also located in multiple geographic regions and climates. The microbial assemblages in leachate samples were characterized through high-throughput sequencing of 16S rRNA gene libraries (bacteria and archaea), combined with a comprehensive analysis of chemical and physical properties. This represents the first comprehensive molecular survey of landfill microbiology.