The role of post-market monitoring (PMM) in the safety assessment of novel foods is critically discussed in order to derive guidelines as to in which situations the application of PMM might be warranted.  Available data sources on food consumption and health status, and the methodologies for generating such data are reviewed.  The paper suggests improvements to make them more applicable for PMM purposes.  It is concluded that any PMM programme must be a hypothesis-driven scientific exercise.  PMM can have a role as a complement to, but not as a replacement for, a comprehensive pre-market safety assessment.  Its use may be appropriate to confirm that product use is as predicted in the pre-market assessment; to provide reassurance that effects observed in the pre-market assessment occur with no greater frequency or intensity in the post-market phase than anticipated; and to investigate the significance of any adverse effects reported by consumers after market-launch.  However PMM is insufficiently powerful to test the hypothesis that any effects seen in the pre-market assessment are absent in the post-market phase.  Current methodologies place limitations on what PMM can achieve.  PMM should only be used when triggered by or when the focus is on specific evidence-based questions.