This paper offers an introduction in religious market theory on the basis of the theme of conversion. Conversions have everything to do with the religious market. Where people are looking for religious satisfaction, they will turn themselves to religious organisations that are willing to give that on certain conditions. Starting from the assumption of the rational actor, the theory makes some strong hypotheses on religious organisations and the religious market. What does the religious market look like, what are the basic characteristics of this market, and how can religious organisations interact with that market? However, when we discuss social-scientific research on conversion in Europe, the limits of religious market theory come to the fore. In the conclusion, the main questions that arise from religious market theory for the project Conversion Careers and Culture Politics in Pentecostalism are being discussed.