The Oxford Companion to Secularism provides a timely overview of the new multidisciplinary field of secular studies. This field involves philosophy, the humanities, intellectual history, political theory, law, international studies, sociology, psychology, anthropology, education, religious studies, and additional disciplines, all showing an increasing interest in the multifaceted phenomenon known as secularism. Conflicts and debates around the world more and more frequently involve secularism. National borders and traditional religions cannot keep people in tidy boxes anymore, as political struggles, doctrinal divergences, and demographic trends are sweeping across regions and entire continents. Simultaneously, there is a resurgence of religious participation in the politics of many countries. How might these diverse phenomena be interrelated, and better understood? As the history of the term “secularism” shows, it has long been entangled with many related issues, such as unorthodoxy, blasphemy, apostasy, irreligion, religious criticism, agnosticism, atheism, naturalism, earth-centered -isms, humanism (and trans- and posthumanisms), rationalism, skepticism, scientism, modernism, human rights causes, liberalism, and various kinds of church–state separation all around the world. Secularism’s relevance also continues to grow due to the dramatic rise of irreligion and secularity in most regions of the world. These trends are leading more and more scholars from a variety of disciplines to investigate secular life and culture in all its varied forms.