Almost everyone uses a cell phone, and almost no one thinks it is dangerous. Cell phones have transformed business and social activities in ways that few people could have predicted 15 or 20 years ago. The technology is now completely embedded in our social fabric. But as with any new technology, safety concerns have been raised. Although clear scientific evidence of cancer risk is lacking, some governments have issued directives to control cell phone use. Such policies have only served to heighten public fears but not to the point that cell phone use has diminished. The public health impact of cell phones serves as a powerful case study to illustrate themes in this book related to risk assessment and risk management. What constitutes a public health threat? How well can we measure small risks when experiences and direct observations are limited? Should a precautionary approach to risk management be adopted when evidence of risk is lacking? The government of the United Kingdom (U.K.) has issued a directive that children up to the age of about 16 should avoid using cell phones as a precautionary measure. Invoking the precautionary principle in this way has created serious questions about management of essentially phantom risks.