SpeciEcity can be imparted to subcellular regions by the setting and subsequent localization of proteins and RNA Although the study of RNA trafficking is in its infancy, the process has been documented already in large germ cells and in asymmetric differentiated cells where visualization of preferential RNA compartmentalization is feasible. An RNA species may be localized for various reasons: it may form a translational point source for a morphogenic gradient (e.g. bicI its partitioning may facilitate inheritance by a subset of cells in embryogenesis (e.g. Vgl mRNA); or it may confer differentiated properties on one part of a cell (e.g. MAP 2 mRNA). The intent of this review is to describe the RNAs that are known to be localized post-transcriptionally, summarize our present knowledge of the molecular mechanisms governing the process, and suggest future directions for this emerging Eeld. The Erst part of this review summarizes the known localized RNA species while the latter two sections focuspdmarily on bi&and Vgl, the two RNAs whose localization has been investigated in detail.