In Switzerland, national census data imply that 84.5 percent are affiliated with religious groups and national survey data imply that 15 percent attend a service on a given weekend in 2009. Research in other countries has shown that attendance is often over-reported, but the level of over-report varies across countries. Thus, in the United States, polls show high and stable levels of religious practice, while Church participation surveys indicate rates twice lower. This article assesses the validity of self-reported affiliation and attendance in Switzerland using the 2009 Swiss National Congregations Study (NCS). Results confirm the observation offered by others that the Swiss census undercounts the persuasions absent from the stylized response categories and they show for the first time that Swiss probably over-report their church attendance, though not as much as in the United States.