Abstract: Mass transport is an important component of any chemical reaction. While typically considered in the realm of chemical engineering for high reaction efficiency and throughput, mass transport at the microscopic level also has profound impacts on catalysis's feasibility and reactivity. Here I present an introduction of the concepts and examples of microscopic mass transport in the context of catalysis. I will offer examples in biology and synthetic catalysts, as well as discuss the physical picture and the mathematical foundations of controlling mass transport at the microscopic level. This presentation will offer some guidance on the creation of microenvironments and the control of spatial concentration profiles for reactants, intermediates, and products in integrated catalysis.