The ciliated protozoa are important members of the interstitial biotope by virtue of their numbers and biomass. These single celled organisms form complex communities in the spaces between sandgrains in many marine sediments. Despite their important role in benthic ecology, there has been a lack of detailed studies on this large and fascinating group. Although difficult and demanding to work with, the most important aspect that has hampered research on these fragile organisms has been the lack of a suitable identification guide.