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Religious diversity is often captured in "mapping studies" that use mostly qualitative methods in order to map and assess the religious communities in a given area. While these studies are useful, they often present weaknesses in that they treat only limited geographic regions, give limited possibilities of comparing across religious groups and cannot test theories. In this chapter, we show how a census and a quantitative National Congregations Study (NCS) methodology can be combined in order to map and assess the religious diversity of a whole country (Switzerland), overcoming the problems mentioned above. We show the methodological steps and selected results concerning organizational, geographic, structural, and cultural diversity.