Fresh water demand is driven by human consumption, agricultural irrigation, and industrial usage and continues to increase along with the global population. Improved methods to inexpensively and sustainably clean water unfit for human consumption are desired, particularly at remote or rural locations. Heterogeneous catalysts offer the opportunity to directly convert toxic molecules in water to nontoxic products. Heterogeneous catalytic reaction processes may bring to mind large-scale industrial production of chemicals, but they can also be used at the small scale, like catalytic converters used in cars to break down gaseous pollutants from fuel combustion. Catalytic processes may be a competitive alternative to conventional water treatment technologies. They have much faster kinetics and are less operationally sensitive than current bioremediation-based methods. Unlike other conventional water treatment technologies (i.e., ion exchange, reverse osmosis, activated carbon filtration), they do not transfer contaminants into separate, more concentrated waste streams. In this Account, we review our efforts on the development of heterogeneous catalysts as advanced reduction technologies to treat toxic water contaminants such as chlorinated organics and nitrates. Fundamental understanding of the underlying chemistry of catalytic materials can inform the design of superior catalytic materials. We discuss the impact of the catalytic structure (i.e., the arrangement of metal atoms on the catalyst surface) on the catalyst activity and selectivity for these aqueous reactions. To explore these aspects, we used model metal-on-metal nanoparticle catalysts along with state-of-the-art in situ spectroscopic techniques and density functional theory calculations to deduce the catalyst surface structure and how it affects the reaction pathways and hence the activity and selectivity. We also discuss recent developments in photocatalysis and electrocatalysis for the treatment of nitrates, touching on fundamentals and surface reaction mechanisms. Finally, we note that despite over 20 years of growing research into heterogeneous catalytic systems for water contaminants, only a few pilot-scale studies have been conducted, with no large-scale implementation to date. We conceive of modular, on- or off-grid catalytic units that treat drinking water at the household tap, at a community well, or for larger-scale reuse of agricultural runoff. We discuss how these may be enhanced by combination with photocatalytic or electrocatalytic processes and how these reductive catalytic modules (catalytic converters for water) can be coupled with other modules for the removal of potential water contaminants.