The control of growth and differentiation is a central question not only for developmental biologists but increasingly for medical research as well. The freshwater polyp hydra was one of the first organisms to be used as a model system for the study of this question. It was chosen because of its simple body plan and because it is made up of only seven to eight different cell types. Recent research has shown that despite their simple body plan, cnidarians already exhibit an impressive repertoire of molecular tools which are responsible for the control of growth and differentiation and amongst which peptides appear to play an important role.