Corynebacteria are widespread and can be detected in clinical and subclinical mastitis as well as in the milk of udder healthy cows. In detail microbiological characteristics, diagnostic methods and mastitis prevalence are illustrated. The pathogenicity of this bacterial group cannot be clearly classified based on the available knowledge. Corynebacteria are therefore classified as mastitis pathogens, commensals and protective microorganisms as well as contaminants. Post milking teat disinfection contributes to decreased new infections. There is still a substantial need for research, especially to determine the species of the present corynebacteria, in order to gain an insight into the relevance of the respective species for udder health.