Hydra are small aquatic organisms found primarily in bodies of unpolluted fresh water. They are only a few millimetres long and are therefore best studied with a microscope. They are used widely as model organisms in developmental biology to study aging, cancer, regeneration and stem cells. Like all models they have their advantages and disadvantages as will be discussed, but generally hydra are an impressive system for the study of the molecular and genetic basis of the above areas. Some species are essentially immortal, so can be maintained in a living state indefinitely.