Rotifers (Rotifera) are microscopic animals with about 1,000 cells. Most are around 100-500µm long and are common in freshwater throughout the world with a few saltwater species. There are about 2,000 different species classified into three classes.

"Rotifera" is derived from Latin and means "wheel-bearer". They get their name from the corona at the top of their head, which is composed of several ciliated tufts around the mouth that, when in motion resemble a wheel.

The cilia are used both for locomotion and to create a current that sweeps food into the mouth. Most free-living forms have pairs of posterior toes to anchor themselves while feeding.

There are a variety of different shapes of rotifers. There is a well-developed cuticle which may be rigid, giving the animal a box-like shape, or flexible, giving the animal a worm-like shape. A few of these move by inch-worming along the substrate. Other rotifers are sessile, living inside tubes or gelatinous holdfasts, and may even be colonial.

The closest relatives to the rotifers are the Acanthocephala (spiny-headed worms).

Rotifers are very important in ecological systems because of their high reproductive rates. They play important roles in energy flow and nutrient cycling, accounting for more than 50% of the zooplankton production in some freshwater environments. Most species are free-living herbivores, bacteriovores or predators. They, in turn, are fed upon by small fish.