Decades of uranium (U) mining, milling and military use left a legacy of U contamination around the world. The radioactivity and chemical toxicity of U at contaminated sites pose an acute and long-term hazard to human health and the surrounding environment. In order to diminish the risk, in situ bioremediation methods, which contribute to contaminant immobilization, are proposed. Nevertheless, the reported prevalent formation of labile and non-crystalline U(IV) species as a result of microbial U(VI) reduction, in contrast to more stable and crystalline uraninite, undermines the effectiveness of the applied bioremediation. Therefore, a holistic understanding of the controls and mechanisms that govern the formation of non-crystalline U(IV) in the environment is at the core of this thesis. Presence of common groundwater solutes (sulfate, silicate and phosphate) were shown to induce the production of bacterial extracellular polymeric substances (biofilm matrix components), which in turn increases the formation of non-crystalline U(IV) as a result of microbial U reduction. In contrast, a field study suggested that non-crystalline U(IV) was a product of abiotic U reduction followed by the sequestration of U(IV) ions by the biofilm matrix. Those contrasting theories, motivated us to look for an indicator capable of differentiating between biotic and abiotic U reduction in the environment. Uranium isotope fractionation proved to be an excellent tool. Based on our results, the isotopic signature of biotic U reduction (accumulation of 238U in the reduced phase) is easily distinguishable from the abiotic U reduction signature (either no isotopic fractionation or fractionation in the opposite direction). When contrasted with U isotope signatures recorded in the sediments, the findings of this study indicated that biological activity contributed to the formation of many ancient and modern U(IV) deposits. Equipped with a tool capable of assessing the origin of the U(IV) product, and with an understanding of the relationship between the biofilm matrix and the formation of non-crystalline U(IV), we embarked on a project to validate the mechanism responsible for non-crystalline U(IV) formation in the subsurface. Results show that regardless whether U is reduced enzymatically or though abiotic agents, the formed U(IV) species are always non-crystalline if the biofilm matrix is present. However, according to our experiments, biological U reduction resulted in better immobilization and overall reduction of U. The mechanism of U reduction in our systems was shown to be controlled by U speciation, which in turn, is govern by the presence of bicarbonate. Under low carbonate concentrations, the majority of U(VI) adsorbs onto the mineral phase thus promoting abiotic U reduction mediated by redox active minerals. However, at higher carbonate concentrations, most of U(VI) is present as aqueous U(VI)-carbonate complexes precluding sorption and thus allowing for enhanced enzymatic U reduction. The results obtained here shed light on the processes occurring at U-contaminated sites and have several field implications. The effectiveness of applied bioremediation was shown to depend on the geochemical conditions at the field site. However, the injection of bicarbonate during in situ clean-up operation enhances microbial U reduction and by this, the overall reductive immobilization of U.