This chapter offers an elementary description of the general aspects of the microorganisms, needed for a better understanding of the microbiological features of biocorrosion. These aspects are classification, cellular structure, metabolism, growth, main species of microorganisms related to corrosion, and different ways of microorganism participation in the corrosion reaction. Although all microbial cells have many common constituents, there are two main basic types, differring in several distinctive features, of cellular structures: prokaryotic and eukaryotic cells. In biological corrosion, the supply of minor amounts of nutrients by the environment can allow a certain microorganism to change that environment from inert to aggressive. Sulfate-reducing bacteria (SRB) play a very important role in biological corrosion. SRB are heterotrophic bacteria, thus requiring an external source of organic carbon, and the energy for their metabolism is provided by the reduction of sulfate to sulfide. Microorganisms are able to actively change the environment surrounding the metal surface to facilitate the corrosion process.