This report looks at average educational levels among adherents of five major world religions – Christianity, Islam, Hinduism, Buddhism and Judaism – as well as among the religiously unaffiliated. Educational systems vary enormously around the world; this report does not attempt to analyze differences in educational quality, but focuses primarily on educational attainment in terms of number of years of schooling. It distinguishes among four broad levels of educational attainment: no formal schooling (less than one year of primary school), primary education (completion of at least one grade of primary school), some secondary education (but no degree beyond high school) and post-secondary education (completion of some kind of college, university or vocational degree beyond high school, also referred to in this report as “higher education”). For comparability across countries, these educational categories are based on the International Standard Classification of Education (ISCED 1997; see Methodology for more details).