Technical Abstract: Environmental microorganisms are diverse and typically exist in dynamic community structures that are continually changing based upon many environmental factors including nutrient levels, temperature, water content, pH, and oxygen status. The emerging microarray technology has already been used in the field of environmental microbiology to estimate microbial diversity, uncover metabolic activity of specific communities, understand interactions between microorganisms and their environment, and identify specific microorganisms. The primary use of microarrays is as a method for comparative quantification of gene expression levels between a control sample and a treatment sample. This review article provides an introduction into the technology focused on the field of environmental microbiology.