In general, corrosion is estimated to account for about 3% of global GDP due to the destruction and replacement costs of steel infrastructure. While it’s challenging to estimate precise costs specific to microbially influenced corrosion (MIC), it’s reasonable to say that globally, MIC costs billions of dollars annually. Several reviews and monographs describe the fundamental processes of MIC that cause accelerated corrosion of steel. Since the introduction of the concept of MIC, certain types of microbes have been associated with the process. The sulfate-reducing bacteria (SRB) are the primary group, as their anaerobic metabolism can produce corrosive metabolites and electrochemical conditions that promote MIC. Methanogenic archaea are another group that is receiving increased attention for creating cathodic conditions on steel surfaces, which can enhance corrosion. Despite being physiologically and genetically distinct, both groups can play a role in MIC given the right conditions. There is still much to learn about the ecology of the MIC process as a whole, including specific metabolic pathways that influence MIC and the identification of key organisms, especially in natural settings.