The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different undulating pattern than flagella. Cilia occur in all members of the group (although the peculiar Suctoria only have them for part of their life-cycle) and are variously used in swimming, crawling, attachment, feeding, and sensation.

Ciliates are an important group of protists, common almost anywhere there is water in lakes, ponds, oceans, rivers, and soils. About 4,500 unique free-living species have been described, and the potential number of extant species is estimated at 27,000-40,000. Included in this number are many ectosymbiotic and endosymbiotic species, as well as some obligate and opportunistic parasites. Ciliate species range in size from as little as 10 µm in some colpodeans to as much as 4 mm in length in some geleiids, and include some of the most morphologically complex protozoans.

In most systems of taxonomy, "Ciliophora" is ranked as a phylum under any of several kingdoms, including Chromista, Protista or Protozoa. In some older systems of classification, such as the influential taxonomic works of Alfred Kahl, ciliated protozoa are placed within the class "Ciliata" (a term which can also refer to a genus of fish). In the taxonomic scheme endorsed by the International Society of Protistologists, which eliminates formal rank designations such as "phylum" and "class", "Ciliophora" is an unranked taxon within Alveolata.

Ciliates have one or more macronuclei and from one to several micronuclei. The macronuclei control metabolic and developmental functions; the micronuclei are necessary for reproduction. Reproduction is typically asexual, although sexual exchange occurs as well. Asexual replication is usually by transverse binary fission or by budding (q.v.). Sexual phenomena include conjugation (genetic exchange between individuals) and autogamy (nuclear reorganization within an individual). Sexual reproduction does not always result in an immediate increase in numbers; however, conjugation is often followed by binary fission.

Although most ciliates are free-living and aquatic, such as the Paramecium (q.v.), many are ectocommensals, dwelling harmlessly on the gills or integument of invertebrates, and some, such as the dysentery-causing Balantidium (q.v.), are parasitic. For information on representative ciliates, see peritrich; spirotrich; suctorian. Ciliates are ubiquitous commensals in the gut of sea urchins (Berger, 1964). Farmanfarmaian and Phillips (1962) stated protozoan in addition to bacterial enrichment occurred in the gut of S. purpuratus after feeding but provided no data. Lasker and Giese (1954) reported several hundred ciliates.

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