In this St Cross Special Ethics Seminar, Anne Schwenkebecher discusses morally wrongful collective inaction and the problem of group-based ignorance. Some of the many things that we could do together with others but fail to do are morally wrongful inactions. While the list of our – individual and collective – non-actions is infinite, not everything that I (or we) fail to do is some form of inaction that is plausibly attributable to me (or us). ‘Collective inaction’ is the unintended failure of two or more agents to perform a collective action or produce a joint outcome where that action or outcome was collectively feasible and where the individual agents had group-based reasons to perform (or produce) it. In a second step we will discuss the role that ignorance plays in excusing morally wrongful collective inaction. We identify three different kinds of collective knowledge (common, pooled, or public) and corresponding types of group-based ignorance. We conclude by showing that inaction is excusable where ignorance sufficiently weakens agents’ group-based reasons for action. Creative Commons Attribution-Non-Commercial-Share Alike 2.0 UK: England & Wales; http://creativecommons.org/licenses/by-nc-sa/2.0/uk/