Sialidases are very common in biological systems. They are found particularly in diverse virus families and bacteria, but also in protozoa, some invertebrates and mammalian. The enzymes differ in their biochemical properties, e.g., kinetics, binding affinity or substrate preference. Nevertheless, they have conserved domains and structural similarities. Their functions offer a wide spread spectrum in nutrition, pathomechanisms and communication in mammalian. In pathogen organisms sialidases give insight in coevolution and interaction between pathogen or commensale and host. As well the enzymes are of interest in drug targeting, like neuraminidase inhibitors of influenza viruses.