A variety of shelled protozoans, belonging to two different Orders of two different Classes, are usually put together into the polyphyletic convenience "group" of “Testacea” or "Thecamoebians". These micro-organisms have been largely ignored by the micropaleontologists due to the assumption that they are too rare to be useful. As fossils, they have been reported irregularly from deposits as old as Neoproterozoic and throughout the Phanerozoic. Their scientific appeal, however, is not in biostratigraphy but rather in paleoecology. Lacustrine and brackish Thecamoebian tests fossilize in numbers comparable to those of their cousins, the foraminifera, so provide large populations in small samples. Recent attempts to use them as paleoecological proxies have been quite successful, proving them to be very inexpensive and practical proxies for the detection of: [a]freshwater/marine transitions such as those in the Everglades, [b] eutrophication, [c] heavy metal pollution, [d] pH indicators and, [e] in one case, as forensic proxies. They are among the very first colonizers of new water bodies and their tests are usually deposited at the bottom of lacustrine sequences where they record pristine conditions, thus providing an ideal baseline for the detection of subsequent environmental changes.