A review of the current state of knowledge on the Monoplacophora, a taxon crucial for assessing phylogeny of the Mollusca, is provided. A number of questions related to this taxon have been resolved; the seriality of some organ systems is now regarded as secondarily gained within mollusks and shared with polyplacophorans; the tergomyan-cyclomyan condition does not reflect a principal difference in organization. Other topics remain to be resolved. These concern the relationships amongst monoplacophorans and the placement of the Monoplacophora within the Mollusca. Furthermore we point out a number of issues to be investigated (e.g., mechanism of excretion or development sequence of gills and muscles, gene expression) by fine structural, ontogenetic, or molecular analyses.