Mycobacterium avium subsp. paratuberculosis (M. paratuberculosis) causes Johne's disease in ruminants (including cattle, sheep and goats) and other animals, and may contribute to Crohn's disease in humans. This possibility, and the fact that M. paratuberculosis may be present in raw milk, make it important to ensure that the heat treatment specified for pasteurization of milk will give acceptable inactivation of this bacterium, with an adequate margin of safety. Published studies of the heat resistance of this bacterium in milk have given widely differing results. Possible reasons for these differences, and the technical problems involved in the work, are reviewed. It is concluded that there is a need (i) for the adoption of an agreed Performance Criterion for pasteurization of milk in relation to this bacterium, (ii) a need for definitive laboratory experiments to understand and determine the heat resistance of M. paratuberculosis, and (iii) a need for an assessment of whether the minimum heat treatments specified at present for pasteurization of milk (Process Criteria) will meet the Performance Criterion for M. paratuberculosis. Measures are also required to ensure that commercial processes deliver continually the specified heat treatment, and to ensure that post-pasteurization contamination is avoided.