Abstract: The International Agency for Research on Cancer (IARC) has classified high as well as low-frequency fields as “possibly carcinogenic to humans” (Group 2B). For high frequency fields the recent assessment is based mainly on weak positive associations described in some epidemiological studies between glioma and acoustic neuroma and the use of mobile and other wireless phones. Also for lowfrequency fields the evidence is based on epidemiological findings revealing a statistic association between childhood leukemia (CL) and low-level magnetic fields. The basic findings are already 10 years old. They have since been supported by further epidemiological studies. However, the knowledge on the main/crucial question of causality has not improved. This fact and in addition the small, but statistically significant increased incidence of CL in the surrounding of German nuclear power plants have motivated the German Office for Radiation Protection (BfS) to work toward a better understanding of the main causes of CL. A long-term strategic research agenda has been developed which builds on an interdisciplinary, international network and aims at clarifying the aetiology of childhood acute lymphoblastic leukemia.