Planktonic microfossils in Proterozoic sedimentary rocks can be recognized on the basis of their facies distribution, morphology, independence of the distributions of benthic microbial associations, and, where observable, spatial distribution within a rock sample as seen in thin section. Like their Phanerozoic counterparts, late Precambrian plankters exhibit discernible inshore-offshore patterns of distribution. Inshore and, especially, lagoonal biotas are characteristically low diversity assemblages dominated by one or a few taxa, while contemporaneous plankton assemblages from open shelf environment contain a diverse heterogeneous array of morphologically complex forms. As the paleoecological ranges of Proterozoic plankters are discernible, so are their stratigraphic ranges, and this permits the establishment of a series of microfossil assemblage zones that have proven to be of considerable value in correlating Upper Riphean and Vendian sequences and establishing the position of the Precambrian/Cambrian boundary within sedimentary successions. Because it is possible to observe diversity distributions within a single time plane and biostratigraphic distributions in successive time intervals, it is possible t o examine diversity trends through time. The plankton record documents the initial observable radiation of cyst-forming eukaryotic microorganisms during the late Proterozoic and indicates, as well, that the plankton biota suffered major extinctions near the end of the eon. A second radiation of baltisphaerid and other morphologically complex taxa restored high plankton diversity levels, but not until well into the early Cambrian.