the contemporary debate about secularization, once intellectually dormant, is currently alive and well, but unfortunately much of the evidence cited by both sides remains partial and selective. It is difficult to draw systematic generalizations about the vitality of religious life around the globe from studies focused on one or two nations, a limited time period, or a single indicator of secularization. One scholar may examine the evidence of lapsed churchgoing habits in Britain and Ireland since the 1960s, for example, and conclude that secularization is proceeding apace, then another may challenge this by citing the vigorous resurgence of radical Islam in Iran and Algeria during the last decade, the rise of Pentecostal churches in Latin America, the Presbyterian expansion in South Korea, or the existence of ethno-religious conflict in Bosnia-Herzegovina. Many arguments simply point toward the continuing popularity of religion in the United States, as though this exception by itself refutes general patterns worldwide. The process of selecting case studies based on the dependent variable generates more heat than light. A more systematic overview is needed, comparing multiple indicators of religiosity across many cultures and regions of the world. In this chapter we outline the comparative framework used in the book and describe the sources of evidence, the societal classifications, and the measures adopted in this study. Building on this foundation, the next chapter compares the systematic evidence for religiosity and secularization occurring during the last fifty years.