To understand why conversion has become a controversial issue, this contribution examines its different meanings and regulations both in the religious legal systems and in State laws. This exam reveals the analogies of the religious legal mechanisms governing the adoption and change of a religion and how they can influence State rules. The different impact that conversion may have on the enjoyment of civil and political rights explains why in some countries conversion is unregulated by the State while in others is subject to controls and limitations. The article concludes that freedom of religion does not require that States refrain from regulating conversion always and everywhere. In some circumstances, and provided that freedom to change religion is not affected, some degree of State control is required by the fact that the legal status of citizens is affected by conversion.