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