A broad definition of what constitutes a sectarian group is social and theological deviance and/or innovation, often paired with some level of critique of and isolation or withdrawal from society. It is impossible to say how many children (under the age of 18) worldwide grow up in so‐called sectarian groups. There are primarily three reasons why such an estimation is problematic: the definitions of the terms new religions, cults, and sectarian groups vary according to time and location; the nature of sectarian groups tends to encompass withdrawal from society, which means that the beliefs of a group may not be known to wider society; and the groups are often wary about stating or have no knowledge of their own membership figures, sometimes due to unclear definitions of membership. This entry examines various impacts of the ways in which sectarian groups interact with socialization, education, health, and abuse, and the possible consequences for children reared in such groups.