As this handbook sets out to explore, the question of leaving a religious tradition is a common question and a potential problem within all religious traditions in both past and present. To draw up a line between insiders and outsiders and to argue that one’s interpretation of the religious tradition is right and that one’s opponents are wrong (for example by calling the other group heretics, or apostates) is therefore a general pattern that is found in all social formations that make use of a religious vocabulary. The handbook on leaving religion consists of three sections covering: (1) Major debates about leaving religion; (2) Case studies and empirical insights; and, finally, (3) Theoretical and methodological approaches.