A total of eight species and three hybrids are currently accepted in the genus. A few botanists treat the genus as just a single variable species (for which the European name N. lutea has priority), while 10–12 or more species are accepted by some other authorities. Recent molecular work has shown that there are substantial differences between the Eurasian species (sect. Nuphar) and American species (sect. Astylus), except for North American N. microphylla which is clusters with the Eurasian species.
The genus is closely related to Nymphaea. Nuphar differs in having its petals being much smaller than its 4-6 bright yellow-coloured sepals, whereas in Nymphaea, the petals are much larger than the sepals. The fruit maturation also differs, with Nuphar fruit being held above water level to maturity, whereas Nymphaea fruit sink below the water level immediately after the flower closes. Both genera share leaves with a radial notch from the circumference to the petiole (leaf stem) in the centre.