Schiedea (Caryophyllaceae subfam. Alsinoideae), endemic to the Hawaiian Islands, comprises deciduous perennials, sprawling subshrubs, shrubs, and rainforest vines exhibiting an extraordinary range of breeding system variation and habitat preference. Morphological and molecular phylogenetic analyses support the division of Schiedea into seven sections and 34 species (including two subspecies). In a majority of the species each floral nectary is either terminated by a tubular, straight structure (a shaft) that extrudes a drop of nectar at the tip, or in three species a recurved shaft that deposits a pool of nectar on the adaxial face of each sepal. In four species (sections Alsinidendron and Nothoschiedea, formerly treated as a separate genus, Alsinidendron) the nectary appendages are flap- or cup-like, and collect large quantities of black nectar. These hypodermic shafts and flap- or cup-like extensions of the floral nectaries are unique within the Caryophyllaceae, and serve as a key feature to delineate Schiedea as a monophyletic group resulting from a single colonization of the Hawaiian Islands. Species are hermaphroditic and insect- or bird-pollinated or autogamous, or dimorphic (dioecious, subdioecious, and gynodioecious) and wind-pollinated. Nearly all hermaphroditic species occur in mesic or wet forests, whereas all dimorphic species are found in dry, often windy habitats. The morphological changes associated with the evolution of wind pollination, including a shift to smaller ﬂower size, abundant pollen production and high pollen/ovule ratios, smaller pollen grains, and upright, highly condensed inflorescences, have had profound effects on the morphological diversity found within this lineage. Two species (S. amplexicaulis, S. implexa) are considered extinct, and the others are greatly at risk. Three new combinations (Schiedea sect. Alsinidendron, S. diffusa subsp. macraei, S. obovata) are proposed, and five new sections (S. sect. Alphaschiedea, S. sect. Anestioschiedea, S. sect. Leucocalyx, S. sect. Mononeura, S. sect. Polyneura) and two new species (S. laui, S. perlmanii) are described. All species are illustrated and their ranges mapped.