Imagine that in 1678 you are Christiaan Huygens or Antonie van Leeuwenhoek seeing paramecia swim gracefully across the field of view of your new microscope. These unicellular, free-living, and swimming cells might have remained a curiosity if not for the ability of H.S. Jennings (Behavior of the lower organisms. Indiana University Press, Bloomington, 1906) and T.M. Sonneborn (Proc Natl Acad Sci USA 23:378-385, 1937) to recognize them for their behavior and genetics, both Mendelian and non-Mendelian. Following many years of painstaking work by Sonneborn and other researchers, Paramecium now serves as a modern model organism that has made specific contributions to cell and molecular biology and development. We will review the continuing usefulness and contributions of Paramecium species in this chapter.Even without a microscope, Paramecium species is visible to the naked eye because of their size (50-300 μ long). Paramecia are holotrichous ciliates, that is, unicellular organisms in the phylum Ciliophora that are covered with cilia. It was the beating of these cilia that propelled them across the slides of the first microscopes and continue to fascinate us today. Over time, Paramecium became a favorite model organism for a large variety of studies. Denis Lyn has called Paramecium the "white rat" of the Ciliophora for their manipulability and amenity to research. We will touch upon the use of Paramecium species to examine swimming behavior, ciliary structure and function, ion channel function, basal body duplication and patterning, non-Mendelian cortical inheritance, programmed DNA rearrangements, regulated secretion and exocytosis, and cell trafficking. In particular, we will focus on the use of P. tetraurelia and P. caudatum.