Microorganisms are able to drastically change the electrochemical conditions at the metal/solution interface by biofilm formation including bacterial consortia and extracellular polymeric substances (EPS) as the main components. The presence of biofilms generally facilitates the initiation of localized corrosion but this effect can be reversed to corrosion inhibition. Microbial corrosion inhibition and its counter process, microbiologically influenced corrosion (MIC) are rarely linked to a single mechanism or to a single species of microorganisms. In recent years microbial inhibition of corrosion and protection of metals have been attributed to the blocking effect of biofilms and EPS. However, this simplistic approach must be discussed from an electrochemical point of view to understand the complex metal-microbe interactions at the metal surface. With this aim, several practical cases involving sulfate-reducing bacteria (SRB) and other microorganisms are critically discussed. General mechanisms for interpreting the influence of microorganisms on the protection and passivity of metals are presented.