In the literature on “moral mathematics,” one issue is whether, and if so to what extent, it is wrong to cause a tiny harm to each of a large number of people, and in particular whether doing so could ever be as seriously wrong as causing a substantial harm to one person.  My topic will be the closely related issue of proportionality in defense against those who would inflict only tiny harms.  For example, might a person who would otherwise inflict a tiny harm on each of a large number of people be liable to be killed in defense of those people?  I will suggest that such a person seems liable to be killed in some cases but not in others, depending on what other people might be doing or on other facts about the context in which the harms would occur.  I will review a range of examples involving the infliction of tiny harms that reveal some surprising facts about the conditions and limits of liability to defensive harm.