The international religious data in the World Christian Database (WCD), and its print predecessor, theWorld Christian Encyclopedia (WCE) have been used frequently in academic studies and the popular press. Scholars have raised questions about the WCD's estimates categories, and potential bias, but the data have not yet been systematically assessed. We test the reliability of the WCD by comparing its religious composition estimates to four other data sources (World Values Survey, Pew Global Assessment Project,CIA World Factbook, and the U.S. Department of State), finding that estimates are highly correlated. In comparing the WCD estimates for Islamic countries and American Christian adherents with local data sources, we identify specific groups for which estimates differ. In addition, we discuss countries where the data sets provide inconsistent religious estimates. Religious composition estimates in the WCD are generally plausible and consistent with other data sets. The WCD also includes comprehensive nonreligious data. Recommendations regarding the use of the WCD are given.