Nautilus are the most primitive of living cephalopods, with primitive eyes lacking a lens (comparable to a pinhole camera) and about ninety relatively short tentacles surrounding a mouth that bears a chitinous beak.

The Nautilidae taxonomic family has existed since the Triassic period and most of its species are now extinct, today it comprises three living species in two genera, although the answer to "how many species are there?" depends on whom is being asked.

According to some, Nautilus pompilius is only one of more than six species in the genus Nautilus, there is now also a second genus in the family: Allonautilus, with one or two species (depending on whom is asked), however according to people closely working with the Nautilus, there are only two distinct species of Nautilus (N. pompilius and N. macromphalus) and one species of Allonautilus (A. scrobiculatus). All the other named "species" of Nautilus would then be subspecies, or even simply geographical variations, the differences in the shells, if any, as well as in the animals themselves do not appear to be sufficient to warrant a species designation.