This presentation is a brief description of neuraminidases and is mainly concerned with the substrate specificity of these enzymes. From the observed great differences in substrate specificity it is clear that we are dealing not with one enzyme called neuraminidase but with a variety of enzymes named neuraminidases. Further investigations in this direction should clearly define and make known the properties of these enzymes. Some problems of the biological function of viral neuraminidases have been reviewed recently (Drzeniek, 1972). Interesting aspects of the specific role of sialic acids were uncovered by the use of the substrate specificity of viral and bacterial neuraminidases (Ray & Simmons, 1972). It is hoped that our knowledge on neuraminidases will stimulate their use in the fields of cytology and histochemistry and enable us to understand the biological function of neuraminic acid.