Diagnosis of mastitis can be carried out precisely on the basis of detection of pathogens and inflammatory reactions. The diagnosis of the risk as such is much more complex, especially because not only the udder related criteria, but also the general health status of the cow must be included in the analysis to get a holistic view. The permanent and unavoidable contamination with a variety of mastitis pathogens requires the estimation of the mastitis risk determined by cow related factors to ensure an economic and effective control of mastitis. The defence potential against new infections consists of local mechanisms such as teat canal barrier, protective function of epithelial cells and cellular as well as humoral milk constituents. Furthermore, systemic immunological reactions are included and enable the specific recognition of antigens and comprise phagocytic and neurohumoral mechanisms. The efficiency of the defence potential against mastitis pathogens is mainly determined by the physiological equilibrium of the cow (homoeostasis). Besides the immune system the neural and the endocrine system is involved in the protection of the defence potential of the individual cow. Therefore, an increased risk for new infections exists in periods of highest yields (e. g. early lactation). This failure in immune functions is mainly due to interrelations between metabolic disturbances and phagocytic defences.