The genus Nuphar (Nymphaeaceae) is described to include 11 species native to fresh waters of the Northern Hemisphere. This assessment utilized morphometric analyses, cladistic analyses, evaluation of randomly amplified polymorphic DNA, and seed and pollen fertility estimations. Evolutionary reconstructions position the genus at the base of the family, most closely allied to Barclaya. Comprehensive cladistic analyses of morphological and molecular data have permitted a general phylogenetic interpretation of the genus. Two major evolutionary lineages are evident and recognized taxonomically at the section level. One (sect. Nuphar) includes N. lutea, N. × spenneriana, N. japonica, and N. × saijoensis, with an ultimate branch involving N. pumila and N. microphylla. The other lineage (sect. Astylus) includes N. polysepala, N. × rubrodisca, N. variegata, N. sagittifolia, and N. advena. All studies illustrate high morphological variability and close relationships of species within each section. The biology of Nuphar is summarized and a complete taxonomic treatment is presented.