Foraminifera are traditionally defined as marine granuloreticuloseans characterized by the presence of a membraneous, agglutinated or calcareous test. This definition has been recently challenged by molecular phylogenetic studies which showed that Foraminifera include both testate and naked species and that they occur in marine, freshwater, and terrestrial environments. Molecular data also revealed high taxonomic diversity of monothalamous (single-chambered) foraminiferans that developed different types of organic and agglutinated tests. First analyses of ribosomal DNA sequences suggested an early divergence of Foraminifera in the evolutionary history of Eukaryotes, but this result was not confirmed by later protein sequence data. Furthermore, analysis of variable regions in ribosomal DNA revealed the presence of several cryptic species, whose geographic distribution seems to be related to oceanic water mass circulation and productivity.