Foraminifera is a big group of unicellular eukaryotic and mostly sea animals, having a hard shell of complexity striking for unicellular organisms. It includes over 4,500 genera and about 40,000 species, mostly now extinct. Their usual size is about 0.5 mm but some giant forms may reach 17 to 20 cm. The multi-chamberedness of their advanced forms with their complex inner integrative systems represents the evolutionary attempt to overcome a unicellular level of organization. The role of the ramified system of the thinnest canals in their multi-chambered shells could be compared with the role of the blood system in Metazoans. The beauty of their shells attracted Leonardo da Vinci to describe them in his ornaments. The highest morphological variability of their shells along with the small number of their chromosomes may permit their usage in the future as classical objects of genetic-morphological studies. Their species are used as bio-indicators of recent and paleo environments, as well as markers of the carbohydrates bearing strata.