There are many cases in which people collectively cause some morally significant outcome (such as a harmful or beneficial outcome) but no individual act seems to make a difference. The problem in such cases is that it seems each person can argue, ‘it makes no difference whether or not I do X, so I have no reason to do it.’ The challenge is to say where this argument goes wrong. My approach begins from the observation that underlying the problem and motivating the typical responses to it is a standard, intuitive assumption. The assumption is that if an act will not make a difference with respect to an outcome, then it cannot play a sig- nificant, non-superfluous role in bringing that outcome about. In other words, helping to bring about an outcome requires making a difference. I argue that the key to solving the problem is to reject this assumption. I develop an account of what it is to help to bring about an outcome, where this does not require making a difference, and I use this explain our reasons for action in the problem cases. This account also yields an error theory that explains why the standard assumption is so tempting, even though it is mistaken.