Heat treatment is the most frequently applied process to inactivate e.g. (pathogenic) micro-organisms and to ensure the quality and safety of milk and milk products. Several research groups have published data on inactivation of Mycobacterium avium subspecies paratuberculosis (MAP) in milk under various pasteurization conditions. Different conclusions about the heat resistance of MAP result from the differential experimental conditions such as the history of the MAP cultures, cell clump disruption, heat application, inactivation data acquisition. In the presentation, parameters critical for heat inactivation experiments will be discussed, in particular the influence of clumping of cells. Application of predictive models has confirmed that tailing of the inactivation curve of MAP is most likely due to the presence of cell clumps and not caused by a more heat resistant cell fraction. According to the model perdictions, the higher the fraction of cells in clumps and the larger the cell clumps, the more tailing will occur. The knowledge of critical factors influencing the heat inactivation experiments was used to set up new studies to ascertain the effectiveness of pasteurization for inactivation of MAP. Industrial pasteurization was simulated in a pilot plant using a turbulent flow at high temperatures and short holding times. Under various conditions, the heat resistance of MAP in milk was determined