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Do mobile phones cause cancer? If the answer were to prove to be “yes,” then more than half of the world’s population is already at risk.1 Mobile phone use increasingly begins in childhood and will likely extend across the full life span, if this technology is not surpassed by another mode of communication. From the public health viewpoint, the introduction of mobile phones, now used by some 5.5 billion people, represents a massive new exposure for which safety information is needed.A review carried out at the end of May 2011 by the World Health Organization’s International Agency for Research on Cancer (IARC) and published as an IARC Monograph2,3 gave a first answer to the question. This review classified radiofrequency electromagnetic fields—the type emitted by mobile phones—as “possibly carcinogenic to humans.” Corresponding to Group 2B in the agency’s four-level system, this expresses an inherent uncertainty based on evidence in humans of a positive association between exposure to an agent and cancer “for which a causal interpretation is considered by the Working Group as credible, but bias and confounding could not be ruled out with reasonable confidence.” This classification signals a warning, but without enough evidence to move radiofrequency electromagnetic fields to a higher level of concern (Group 2A-Probable or Group 1-Carcinogenic to Humans).The 2B classification was driven largely by the epidemiologic findings, particularly a set of case-control studies carried out in Sweden by Hardell and colleagues4 and the multicountry INTERPHONE study.5 Difficult methodological problems cloud interpretation of these observational studies,6 and the IARC Working Group concluded that the human studies provided “limited evidence” for carcinogenicity.The 2B classification for radiofrequency electromagnetic fields by IARC continues to receive worldwide media attention, and it remains of great interest to the public, reflecting the increasing use of mobile phones in our lives. There are diverse opinions about this classification, with deep skepticism from those who see no possibility of carcinogenesis by radiofrequency electromagnetic fields based on biophysical principles or from those who find the epidemiologic findings less convincing. For instance, an article by a panel of the International Commission on Non-Ionizing Radiation Protection (submitted in March 2011) had concluded that the evidence weighed against causation.7 An editorial accompanying a case-control study of childhood brain cancer published after the meeting of the IARC Working Group and interpreted as “negative” offered a similar view.8 At variance with this interpretation, another group of scientists regarded the available data as simply “insufficient to make any determinations” about use longer than 10 years.9Here, we offer our views on these studies and the next steps after the IARC classification. This commentary is based on our knowledge of the epidemiologic evidence and our participation (J.M.S. as Chair and R.S., K.S., and J.S. as members of the IARC secretariat) in the IARC Working Group. We make the inevitable call for “more research” but propose an integrated and strategic agenda that will target key uncertainties, well highlighted by the IARC review.The IARC Working Group reviewed most of the literature available on radiofrequency electromagnetic fields up to May 2011. The studies date back decades, reflecting various eras of heightened concern about the consequences of electromagnetic fields—for example, worker exposures to radar, and mobile phones and brain cancer dating from the 1990s. Epidemiologic studies have addressed worker populations with higher whole body exposures than the general population, as well as exposures to the general population. Parallel mechanistic studies have been carried out, largely using in vitro methods.