This paper examines the causes and effects of mobile number portability (MNP) and provides a survey of its implementation in Europe. We first examine the competitive effects and the costs of introducing MNP. Next, we discuss how to charge for MNP. We argue that a price cap regime starting from the average cost of porting is likely to provide appropriate incentives. Finally, we review the 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.