Radiolarians are unicellular, marine animals with opaline skeletons ranging in size from 100 to 400 µm and in excelptional cases up to 2mm. They mostly live in the upper few hundred meters of the water colummn in all oceans of normal salinity, but their diversity and abundance decline markedly in near-shore waters. In high latitudes species, diversity is about one tenth that of the tropics, but individual species abundance is high. The radiolarians are preserved in a variety of soliceous deposits , including chert, dating back to the Paleozoic. Although their great diversity has delayed a satisfactory taxonomy for the majority, precluding recording of the entire assemblage to species level, several well understood evolutionary lineages provide precise biostratigraphic interpretations of the Cenezoic. Poor preservation has hampered pre-Cenezoic radiolarian taxonomy and it is only recently that a Cretaceous biostrtaigraphic zonation has been developed. In addition to their biostratigraphic utility, radiolarian assemblages can provide palaeoenvironmental information such as temperature, water depth, upwelling and surface water productivity in Neogene sediments.