The expression ‘you need to know to conserve’ is a well-known cliche among biologists. Documenting the richness of a group of organisms is the first step towards understanding biodiversity and preparing efficient conservation plans. In this context, many efforts have been made to quantify the number of species on Earth and estimate the number of species still unknown to science. A few countries have complete and integrated databases estimating the approximate number of species recorded for their territory, particularly in the Global South. In Brazil, a country of continental dimensions, revealing the richness of the second most diverse clade of invertebrates (=Mollusca) has been a goal of taxonomists. Recently, in an unprecedented, collective, and integrated effort among Brazilian malacologists, it was possible to estimate how many valid species of molluscs are there in Brazil. In this effort, more than 30 mollusc experts joined together to update the Taxonomic Catalogue of the Brazilian Fauna (TCBF), a governmental website that allows a quick and real-time updating of all Metazoan. So far, more than 5,000 updates have been made in TCBF, indicating the presence of 3,552 valid species of molluscs in Brazil, distributed among the main clades as follows: Caudofoveata (10 spp.), Solenogastres (6 spp.), Polyplacophora (35 spp.), Scaphopoda (43 spp.), Cephalopoda (92 spp.), Bivalvia (629 spp.) and Gastropoda (2,737 spp.). The present study, in addition to demonstrating for the first time the richness of Brazilian molluscs, also presents the state of the art of this important phylum of invertebrates highlighting its most representative and neglected groups.