In this thesis, the bioconversion of palladium and gold solutions and gold-bearing wastes into highly valuable mono- and bimetallic catalysts is described. This process relies on bioreduction; the ability of some bacteria to reduce Pd(II) and Au(III) ions at the expense of an exogenous electron donor with precipitation as zero valent metals. The resulting metallic nanoparticles (NPs) immobilised on the outer membrane and within the periplasm exhibit remarkable catalytic properties, sometimes surpassing commercially available catalyst formulations in terms of activity and/or selectivity. Previous studies in the field have mainly focused on the ability of Desulfovibrio spp. to reduce Pd(II) from both surrogate solutions and reprocessing wastes. The mechanism of Pd(II) reduction in this genus was previously shown to be enzymatic, involving hydrogenases, key enzymes of hydrogen metabolism. In this study, a detailed investigation into the mechanism of Pd(II) reduction by Escherichia coli using a genetic approach confirmed hydrogenase involvement and additionally showed that these enzymes are needed to initiate the formation of Pd(O) nuclei. Genetically engineered strains depleted of all functional hydrogenases lost their ability to produce Pd(O) NPs, which in turn greatly affected the catalytic activity of the resulting bioinorganic catalyst ("bioPd(O)"). Further studies suggested that the nature of the bacterial support also influenced the catalytic activity of bioPd(O) preparations. Seven bacterial strains, representing different Gramnegative and Gram-positive genera, were tested for Pd(II) reduction. Large differences in Pd(II) sorption and Pd(II) reduction ability were observed between strains; the combination of these factors affected the final size distribution of the cell-bound Pd(O) NPs and hence the catalytic activity of the resulting bioPd(O) preparations. Bioinorganic catalysts were shown to be active and/or selective in a wide variety of reactions, including Cr(VI) reduction, hydrogenolysis (reductive dehalogenation), Heck coupling and oxidations. The bioreductive approach was applied to demonstrate Au(III) reduction and recovery using cells of D. desulfuricans and E. coli and the first evidence of the catalytic activity of biogenic Au(O) NPs is presented. Au(III) reduction was slower than Pd(II) reduction and only partially involved hydrogenases which suggested the involvement of an additional different reduction route. However, introducing a bionanocatalyst consisting of lightly pre-palladized cells into the process greatly improved the speed of Au(III) reduction and resulted in the formation of highly ordered AulPd core/shell nanostructures which exhibited catalytic properties not seen with traditional chemical counterparts.