Humus is a ubiquitous component in the environment and can be readily isolated from nearly all soils, waters, and sediments. Humus biodegrades poorly and is formed from the decomposition of plant, animal, and microbial cells in soils and sediments. The functional groups of humus, which determine the physical and chemical characteristics, vary depending on the origin and age of the material. An in-depth understanding of the reaction chemistry of humus and the role played by these compounds in geochemical cycles has been a major scientific focus for over a century. However, the interactions between these compounds and microbial populations have only recently gained scientific attention. It is now known that humic substances may play an important role in the anaerobic biodegradation and biotransformation of organic as well as inorganic compounds. Humus can be utilized by microorganisms as effective electron acceptors for the oxidative degradation of organic carbon in anaerobic environments. Alternatively, reduced humic substances can be utilized by microorganisms as electron donors for the assimilation of organic carbon coupled to denitrification. Microbially reduced humus can act as soluble electron carriers between microorganisms and metal oxides such as Fe(III) oxides, allowing for the regeneration of humus to the oxidized form. Thus, even sub-stoichiometric concentrations of humus can mediate both anaerobic substrate oxidation and metal oxide reduction. Additionally, humus can serve as electron shuttles, abiotically transferring electrons from an external electron donor to priority pollutants, which are susceptible to reductive transformations (e.g., nitroaromatics, azo dyes and polyhalogenated compounds). These interactions may have a significant impact on the fate and transport of organic and inorganic environmental pollutants. An understanding of these interactions may improve strategies for bioremediation of particular pollutants. Likewise, predicting the behavior and longevity of pollutants within the environment may be better modeled, resulting in improved design strategies for bioremediative processes. This paper presented a review on humus respiration which has only recently been recognized as a novel respiratory pathway for a diverse community of humus-reducing microorganisms. Likewise, the role of humic substances on the degradation of priority pollutants was discussed in this paper.