A review on salinity adaptation of marine molluscs based on mainly Russian scientific literature is presented. The existence of two relatively independent systems of adaptation to extreme (resistance level) and moderate (tolerance level) changes of environmental salinity was shown. The resistance of molluscs is based mainly on an impeded water-salt exchange with the external medium due to mantle cavity hermetization. The tolerance of molluscs is determined by cellular mechanisms of adaptation. Reversible changes of protein and RNA synthesis, alteration of the pattern of multiple molecular forms of different enzymes, and the regulation of ionic content and cell volume were shown to be of importance for the above mentioned mechanisms. The efficiency of resistance and tolerance adaptations to salinity changes may vary in different species and in different colour phenotypes of the same species (intrapopulational polymorphism). Parasites (trematodes) may suppress the resistance of the mollusc-host to extreme salinity changes without effecting the host's capacity for adaptive changes in salinity tolerance.