The substantial decrease in solubility accompanying the reduction of U(VI) to U(IV), producing the sparingly soluble mineral uraninite, is viewed as a potential mechanism for the sequestration of environmental uranium contamination. In recent years, a variety of bacteria have been reported to exhibit this reductive capacity. To obtain an understanding of the microbial uranium transformation, to develop a practical approach for the acceleration of in-situ bioreduction, and to predict the long-term fate of environmental uranium, several aspects of the microbial reduction process have been experimentally explored. This dissertation outlines the research that investigates rates of microbial uranium reduction under various experimental conditions, probes the product(s) of enzymatic U(VI) reduction and elaborates the structure of one product –biogenic uraninite– demonstrating the structure- and reactivity-modifying effect of a dopant (Mn(II)) and lastly identifies the products of abiotic uranium reduction by biogenic magnetite and vivianite.