Ceratophyllum is a cosmopolitan genus of flowering plants, commonly found in ponds, marshes, and quiet streams in tropical and in temperate regions. They are usually called hornworts, although this name is also used for unrelated plants of the division Anthocerotophyta.

Ceratophyllum grows completely submerged, usually though not always floating on the surface. They don't withstand drought. At intervals along nodes of the stem they produce rings of bright green leaves, which are narrow and often branched. The forked leaves feel brittle and stiff to the touch. The plants have no roots at all, but sometimes they develop modified leaves with a rootlike appearance, which anchor the plant to the bottom. The flowers are small and don't attract the attention, with the male and female flowers on the same plant.

Because of their appearance and their high oxygen production, they are often used in freshwater aquaria. Hornwort plants float in great numbers just under the surface. They offer excellent protection to fish-spawn, but also to snails, infected with bilharzia.By screening the lighting, they obstruct algal growth.

Hornwort, also known as coontail, is a very easy to grow plant that is tolerant of most water conditions and temperatures. It makes an excellent, tough to kill beginner's plant.

Ceratophyllum is unique enough to warrant its own family, the Ceratophyllaceae, and in newer systems its own order, the Ceratophyllales. It belongs among the basal dicotyledons, and may be the closest relative of the monocotyledons.

The division into species is not completely settled. There are two main species:

• Ceratophyllum demersum : Common Hornwort or (UK) Rigid Hornwort
• Ceratophyllum submersum : Tropical Hornwort

More than 30 other species have been described, but many of them are probably mere variants of these.