ABSTRACT.  Cyperus is a large (ca. 650 species), taxonomically complex genus widely distributed throughout tropical and warm temperate regions of the world.  Many of the species are weedy and four have been ranked among the world's worst weeds.  We will present an overview of the traditional morphologically based systematics of Cyperus, including a synopsis of infrageneric taxonomy and a discussion of the systematic relationships and geographical origins of selected weedy species.  We will explore reasons for inconsistencies in nomenclature and circumscriptions of weedy species and possible solutions and potential problems for new research.