Tetrahymena, a unicellular eukaryote, is a genus of free-living ciliates that can also switch from commensalistic to pathogenic modes of survival. They are common in freshwater ponds. Tetrahymena species used as model organisms in biomedical research are T. thermophila and T. pyriformis.

β-tubulin in Tetrahymena.
As a ciliated protozoan, Tetrahymena thermophila exhibits nuclear dimorphism: two types of cell nuclei. They have a bigger, non-germline macronucleus and a small, germline micronucleus in each cell at the same time and these two carry out different functions with distinct cytological and biological properties. This unique versatility allows scientists to use Tetrahymena to identify several key factors regarding gene expression and genome integrity. In addition, Tetrahymena possess hundreds of cilia and has complicated microtubule structures, making it an optimal model to illustrate the diversity and functions of microtubule arrays.

Because Tetrahymena can be grown in a large quantity in the laboratory with ease, it has been a great source for biochemical analysis for years, specifically for enzymatic activities and purification of sub-cellular components. In addition, with the advancement of genetic techniques it has become an excellent model to study the gene function in vivo. The recent sequencing of the macronucleus genome should ensure that Tetrahymena will be continuously used as a model system.

Tetrahymena thermophila exists in 7 different sexes (mating types) that can reproduce in 21 different combinations, and a single tetrahymena cannot reproduce sexually with itself. Each organism "decides" which sex it will become during mating, through a stochastic process.

Studies on Tetrahymena have contributed to several scientific milestones including:

- First cell which showed synchronized division, which led to the first insights into the existence of mechanisms which control the cell cycle.
- Identification and purification of the first cytoskeleton based motor protein such as dynein.[3]
- Aid in the discovery of lysosomes and peroxisomes.
- Early molecular identification of somatic genome rearrangement.
- Discovery of the molecular structure of telomeres, telomerase enzyme, the templating role of telomerase RNA and their roles in cellular senescence and chromosome healing (for which a Nobel Prize was won).
- Nobel Prize–winning co-discovery (1989, in Chemistry) of catalytic RNA (ribozyme).
- Discovery of the function of histone acetylation.
- Demonstration of the roles of posttranslational modification such as acetylation and glycylation on tubulins and discovery of the enzymes responsible for some of these modifications (glutamylation)

The life cycle of T. thermophila consists of an alternation between asexual and sexual stages. In nutrient rich media during vegetative growth cells reproduce asexually by binary fission. This type of cell division occurs by a sequence of morphogenetic events that results in the development of duplicate sets of cell structures, one for each daughter cell. Only during starvation conditions will cells commit to sexual conjugation, pairing and fusing with a cell of opposite mating type. Tetrahymena has seven mating types; each of which can mate with any of the other six without preference, but not its own.

Typical of ciliates, T. thermophila differentiates its genome into two functionally distinct types of nuclei, each specifically used during the two different stages of the life cycle. The diploid germline micronucleus is transcriptionally silent and only plays a role during sexual life stages. The germline nucleus contains 5 pairs of chromosomes which encode the heritable information passed down from one sexual generation to the next. During sexual conjugation, haploid micronuclear meiotic products from both parental cells fuse, leading to the creation of a new micro- and macronucleus in progeny cells. Sexual conjugation occurs when cells starved for at least 2hrs in a nutrient-depleted media encounter a cell of complementary mating type. After a brief period of co-stimulation (~1hr), starved cells begin to pair at their anterior ends to form a specialized region of membrane called the conjugation junction.

Free swimming cells of Tetrahymena are attracted to certain chemicals by chemokinesis. The major chemo-attractants are peptides and/or proteins.[8]

A 2016 study found that cultured Tetrahymena have the capacity to 'learn' the shape and size of their swimming space. Cells confined in a droplet of a water for a short time were, upon release, found to repeat the circular swimming trajectories 'learned' in the droplet. The diameter and duration of these swimming paths reflected the size of the droplet and time allowed to adapt.