For the last thirty years, the electricity related 50 Hz magnetic fields (MF) have been suspected of being responsible for several pathologies, in particular childhood leukemia [1]. The most recent collective expertise (WHO 2007 and SCENHIR 2009) concluded that the last major interrogation with regard to low frequency MF is the statistical association observed in several joint analyses between the increase of risk of the childhood leukemia and a higher than 0.4 μT exposure to MF on average in a 24-hour period [2]. Currently, the exposure of the French population to these magnetic fields is only approximately known. A study carried out in residences located near high voltage power lines in the "département1 de la Côte d'Or" made it possible to assess the MF background level inside these residences [3]. However, these residences are a limited sample compared to the diversity of the housing developments in France and the study characterized the exposure of the houses and not of the resident people. We are all exposed to many sources of magnetic fields due the fact that we do not remain at home 24 hours a day. Transportation in particular, significantly contributes to the individual exposure. Other places or activities can also constitute sources of exposure such as the workplace, sport activity areas, shopping centers or schools. Should the MF in excess to 0.4 μT on average carry health risk, would the authorities be able to manage it, i.e. estimate the proportion of the French population at risk and identify and mitigate the main sources causing the exposure? To answer this question the Ministry of Health and Solidarities initiated a study on the exposure of a representative sample of the French population to 50 Hz MF. The major issues of this study were to select randomly a representative sample and to collect all of necessary data. Measurements were performed in three campaigns at winter time (October to April). The present paper gives the results of the two first campaigns.