This article deals with an aspect of the special relationship between grandparents and grandchildren: the fosterage of grandchildren by their (classificatory) grandparents. By becoming social parents with full responsibility for their social children, grandparents—as foster-parents—take upon themselves the characteristics typical both of parental roles involving authority, and grand-parental behaviour involving joking, warmth and proximity to their fostered grandchildren. The practices of grandparental fosterage, as well as changes in them, need to be understood in the broader context of widespread fosterage among Baatombu, where the majority of children grow up not with their biological parents but with foster-parents. The gradual decline of old forms of fosterage is affecting foster practices between grandparents and grandchildren. The grandparents’ generation evaluate this decline negatively and fear that changing fosterage patterns may adversely affect their social security in old age.