The history of the species problem as applied to freshwater molluscs, from the beginning of scientific taxonomy to the present day, is outlined. Three main approaches to delineation of species boundaries (intuitive, conceptual, and operational) are discussed, with remarks on their practical usage in freshwater malacology. The central topic of the article is how malacologists changed their views on the essence of species category and the impact of these changes on the taxonomic practice. The opinions of some prominent and prolific workers in the field (Bourguignat, Kobelt, Hubendick, Starobogatov) are analysed as well as the debates around the theoretical foundations and practical results of the ‘Nouvelle École’ of the 19th century and the ‘comparatory’ systematics of the 20th century. It is shown that the operational approach to species delineation is the most popular in current systematic malacology, with strong inclination of practitioners to use reductionist methods.