This paper examines the causes and effects of mobile number portability (MNP) and provides a survey of its implementation in Europe. It first examines the competitive effects and costs of introducing MNP. Next, it discusses how to charge for MNP. It argues that a price cap regime starting from the average cost of porting is likely to provide appropriate incentives. Finally, it reviews recent experience with implementing MNP in Europe. Differences in the speed of porting and porting charges appear to explain part of the differences in the use of MNP across countries.