Created by two of the field's leading experts, this unique introduction to international religious demography outlines the challenges in interpreting data on religious adherence, and presents a contemporary portrait of global religious belief. * Offers the first comprehensive overview of the field of international religious demography - detailing what we know about religious adherents around the world, and how we know it * Examines religious freedom and diversity, including agnostics and atheists, on a global scale, highlighting trends over the past 100 years and projecting estimates for the year 2050 * Outlines the issues and challenges related to definitions, taxonomies, sources, analyses, and other techniques in interpreting data on religious adherence * Considers data from religious communities, censuses, surveys, and scholarly research, along with several in-depth case studies on the global Muslim population, religion in China, and the religious demography of recently created Sudan and South Sudan * Argues against the belief that the twentieth-century was a 'secular' period by putting forward new evidence to the contrary * Provides resources for measuring both qualitatively and quantitatively important data on the world's religious situation in the twenty-first century