The questions of how best to define "descent" and "descent groups" came to the fore once again in the 1950s, in part as a consequence of a great expansion of academic anthropology and of ethnographic fieldwork following World War II, especially in the Pacific, South and Southeast Asia, and Africa. In contrast, affiliation with a nonunilineal descent group, although perhaps in some sense "automatic by birth", is not fixed and immutable but is or can be a matter for individual choice and decision. The practice of describing rules of filiation and affiliation as rules of descent has promoted the practice of describing the groups constituted by those rules as descent groups, especially if the rule is such that the group may not include anyone not "born into" it. This chapter concludes that only groups for inclusion in which matrifiliation is the necessary and sufficient condition can properly be described as "matrilineal descent groups".