This review focuses on the extracellular proteoglycans. Special emphasis is placed on the structural features of their protein cores, their gene organization, and their transcriptional control. A simplified nomenclature comprising two broad groups of extracellular proteoglycans is offered: the small leucine-rich proteoglycans or SLRPs, pronounced "slurps, " and the modular proteoglycans. The first group encompasses at least five distinct members of a gene family characterized by a central domain composed of leucine-rich repeats flanked by two cysteine-rich regions. The second group consists of those proteoglycans whose unifying feature is the assembly of various protein modules in a relatively elongated and often highly glycosylated structure. This group is quite heterogeneous and includes a distinct family of proteoglycans, the "hyalectans," that bind hyaluronan and contain a C-type lectin motif that is likely to bind carbohydrates, and a less distinct group that contains structural homologies but lacks hyaluronan-binding properties or lectin-like domains.