While self care is not always harmful, research finds that when children under 13 are regularly left to spend time alone or to be cared for by young siblings, they may be at risk for injuries and developmental problems. This research brief focuses on two groups of children that may be particularly vulnerable when they lack regular adult supervision: young school-age children and children in low-income families. Following an overview, the research brief looks at the numbers of children in supervised care and in self care, the hours and the circumstances (e.g., family income, child age, parental mental health), and reasons. The brief highlights findings indicating the following: Despite families' limited financial resources, low-income children, in general, were less likely to be in self care than children in higher-income households, especially when young. In addition, children with parents reporting symptoms of poor mental health are more likely to spend time unsupervised than children whose parents do not report such symptoms. Finally, although the vast majority of children are not in self care, the actual number of children in these circumstances is substantial, which may point to unmet needs for affordable supervised care options. (Contains 26 endnotes.) (HTH)