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A cohort is a group of individuals sharing some characteristics, followed longitudinally. Essential tools of epidemiology, these studies provide pieces of evidence of the relationship between an exposition and outcomes in order to guide public health policies. In France, many cohorts have been conducted over the past few years. Sometimes, initiated by independent research teams (e.g. E3N) but more often, either requested by health authorities in a global public health plan (e.g. MEMENTO in the Alzheimer plan) or conducted to investigate a safety issue (e.g. France Coag). Besides, post authorization studies often consist in prospective cohorts. Because of objectives, designs and governance arrangements diversity; participants questioned whether it was interesting for researchers, regulators and industrials to use this epidemiological tool. Some findings about difficulties met in cohorts' establishment have been shared by each other. In order to make cohorts more efficient, participants have made some recommendations.