Sea stars (or starfish) (scientific name Asteroidea) are a major group of the Echinoderms. There are about 2,000 species of sea stars living in the world's oceans in habitats from tropical coral reefs, kelp forests to the cold deep oceans (greater than 6 km), All sea stars are marine animals.

Most sea stars have five arms extending from a central body but some species can can have up to 50 arms. They have an internal skeleton consisting of small bony plates, mainly made from calcium carbonate, covered with spines and granules. The skeleton is covered with skin and has holes in it for the tube feet, mouth and anus.

The larger species can live for over 30 years. Most species are predators eating molluscs (e.g. clams, oysters, mussels and some snails) or other animal too slow to escape them (e.g. other echinoderms or dying fish). Other species are detritivores, eating decomposed animal and plant material. Others consume coral polyps, sponges or even plankton. Many have specialised body parts to hep catch and eat their prey, for example specialized tube feet to extend itself deep into the sand or mud to catch their prey.

Some species of sea star use their water vascular systems to pry open the shells of bivalve molluscs, such as clams and mussels, then injecting their first stomach into the shells. With the stomach inside the shell, the sea star is able to partly digest the mollusc. The stomach is then dranwn back inside the body and digestion is completed.

Despite having complex nervous systems sea stars do not have a true centralized brain. They have senses to detect touch, light, temperature, orientation and chemicals in the water.

The tube feet are also sensitive to chemicals and are where most respiration occurs. The tube feet grip a surface, moving in a wave, with one body section attaching to the surface as another section is released.

Sea stars can reproduce sexually and asexually. In sexual reproduction, fertilization occurs in the water with males and females releasing sperm and eggs into the environment. The fertilized embryos, which are free-swimming animals, become part of the zooplankton in most species. Eventually the larvae undergo metamorphosis, settle to the bottom and grow into adults. Some species brood their eggs, either by simply sitting over them, or using specialised brooding baskets.

Asexual reproduction is by fragmentation, a part of an arm and part of the central disk becomes detached from the "parent" and develops into an independent individual sea star. In the past, many sea stars have been "killed" by chopping them into pieces but they have been able to regenerate and grow into more sea stars. This happend on the Great Barrier Reef in early attempts to control the Crown of Thorn Sea stars.

A sea star's arms can grow to over 60 cm in some species.