A growing body of evidence by new risk assessments concerning permanent exposure to low level, extremely low frequency electric and magnetic fields shows, a moderate long-term risk. This possible long-term risk makes it necessary to think about prudent avoidance in city planning. There is no Federal German legislation to make prevention mandatory. The risk assessments are based on epidemiological investigations of a relationship between residential locations of children in the vicinity of power lines and incidence of leukemia. The study design of recent studies has been essentially optimised compared to previous ones. The investigations have been carried out independently. The overall conclusion is more in favour of a relationship than against it. In Hamburg a preventive practice was established several years ago. Safe distances between power lines and new apartment houses are taken into account. Other concepts of avoidance are presented. These may be guided by values of magnetic fields or can be expressed by security distances, as can be shown by international examples.