All domains of life depend on redox chemistry to generate energy through respiration or fermentation. Electrons flow from electron donors to various terminal electron acceptors (TEAs) through redox active biomolecules. These molecules include proteins (e.g., cytochromes, oxidoreductases) and small molecules (e.g., quinones, flavins, phenazines) and all have the ability to change oxidation states under biological conditions. Oxygen is the most common TEA and is used by all aerobic organisms; however, many microorganisms have evolved to utilize insoluble TEAs, such as metal oxides. Utilization of insoluble TEAs requires passing electrons through cellular membranes and is commonly referred to as extracellular electron transfer (EET). There are three main mechanisms to accomplish EET: 1) direct contact of redox proteins located in the cell envelope with the TEA, 2) secretion of cytochrome containing nanowires or outer membrane vesicles, or 3) use of small molecule redox mediators, also known as extracellular electron shuttles. Of all the EET mechanisms, mediated EET is the least understood with little known about the physicochemical properties of the mediators that drive EET activity.