Photocatalysis is the process where the rate of a photoreaction is altered by the presence of a substance that is not itself consumed in the reaction. Photocatalysis can be heterogeneous or homogeneous, with the majority of the research up-to-date focused on heterogeneous systems such as titanium dioxide (TiO₂). Plasmon-mediated photocatalysis is a new topic in heterogeneous catalysis that shows the potential to increase both the energy efficiency and selectivity of photocatalytic systems. Here we explore the basic physics of plasmons and how they relate to photocatalysis. Next, we review several known examples of plasmon-mediated photocatalytic reactions. We then look at theories explaining the mechanism of plasmon-mediated photocatalysis and some experimental evidence for each mechanism. Lastly, we review the current challenges in understanding plasmon-mediated photocatalysis and possible future research directions.