A collective duty gap arises when a group has caused harm that requires remedying but no member did harm that can justify the imposition of individual remedial duties. Examples range from airplane crashes to climate change. How might collective duty gaps be filled? This paper starts by examining two promising proposals for filling them. Both proposals are found inadequate. Thus, while gap-filling duties can be defended against objections from unfairness and demandingness, we need a substantive justification for their existence. I (...) argue that substantive justification can be found in the normative force of commitments individuals make to others with regard to ends. Along the way, I argue that gap-filling duties must be conceptualized differently in group agents, as compared to non-agent groups: in the former, gap-filling duties can be understood as duties to “take up the slack”; in the latter, this would be a category error. (shrink)