Biofouling and biocorrosion lead to an important modification of the metal/solution interface inducing changes in the type and concentration of ions, pH values, oxygen levels, flow velocity, etc. Metal dissolution in seawater is mainly conditioned by two different processes: (a) biofouling settlement and (b) corrosion products formation. Corrosion-resistant alloys such as stainless steel present an ideal substratum for microbial colonization, rather similar to inert non-metallic surfaces, due to the lack of corrosion products. Stainless steels are sensitive to pitting and other types of localized corrosion in chloride-containing media such as seawater. Biofilms and bacterial metabolism may accelerate the initiation of crevice attack by depletion of oxygen in the crevice solution due to microbial respiration. Bacterial colonization occurs within a period of 24-72 h on stainless steel samples exposed to natural seawater and, depending on environmental conditions, a copious and patchy biofilm is generally formed. Different interpretations of biofilms' effects on corrosion ave critically discussed. A practical case, involving polluted harbour seawater, is reported to illustrate biofilm and corrosion interactions on stainless steel samples.