The widespread adoption of information technology and the use of the Internet in a wide variety of fields, both professionally and personally, has sparked an unprecedented race to collect data of all kinds. This data, whether personal or shared, constitutes what is now known as “big data”—a vast collection of highly diverse information that can be used for multiple purposes depending on the motivations of the companies or organizations exploiting this massive data. In the healthcare sector, the use of big data is a source of both hope (as it has the potential to improve the understanding and resolution of medical problems) and risks (excessive monitoring of individual behavior, ethical issues, etc.), which must be kept in mind to ensure the optimal use of this data for society. Daniel Eilstein and Jérôme Pozuelos present here an overview of the health data currently collected in France, how it is processed, and the issues raised by the protection of personal data. They also highlight the opportunities opened up in the medical field by the use of such big data (particularly what is known as data linkage). Here too, research will have to grapple with the freedom-security dilemma, but provided certain ethical limits are respected, significant advances could be made.