This dissertation examines the taxonomy, morphology, and species relationships of two genera in the family Caryophyllaceae, subfamily Alsinoideae: Pycnophyllum and Pycnophyllopsis. These two genera are restricted to the highest peaks of the South American Andes, from Peru to northern Argentina, above the 3500 m of elevation. Pycnophyllopsis subgen. Coquimbo, a new subgenus and species endemic to northern Chile is proposed based on various morphological characters that confirm its status. A combination of detailed morphological study of herbarium specimens and fieldwork resulted in systematic monographs for these two genera. Eight species of Pycnophyllum are here recognized, with the center of diversity in Peru, where seven species are found. The genus Pycnophyllosis consists of nine species. Pycnophyllopsis smithii a new species from central Peru is described. A molecular phylogeny of the family Caryophyllaceae using nrITS is presented and discussed. All studies species are fully described, illustrated, and mapped. Identification keys are provided for the two larger genera. The systematic relationship with other members of the family is also explored. / text