Microorganisms play a major role in driving the terrestrial selenium cycle by catalyzing the reduction of selenate and selenite in soils and sediments. Although bacteria have been known to reduce selenium oxyanions for nearly 20 years, the biological mechanisms that control the reduction process have remained unknown. By using mutagenesis and direct cloning techniques, I have discovered essential genes that enable bacteria to catalyze selenium reduction. Molecular insight into the biochemical mechanism of selenate reduction was made possible by studying a genetically tractable model organism capable both aerobic and anaerobic growth. This genetic data has allowed me to construct a molecular model for microbial selenate reduction.