Act consequentialism provides an answer to the question of what one ought to do, no matter which situation one is in. The problem, however, is that this answer is rarely if ever known by the agent herself. Ordinary agents do not know all the consequences of their actions, nor do they know how to assess all possible consequences. This lack of both empirical and evaluative knowledge means that ordinary agents often, or always, do not know what act consequentialism tells them to do. This “ignorance challenge, " as we might call it, is often seen as one of the main challenges for act consequentialism. This chapter discusses the main responses to this challenge. It also asks whether this challenge is unique to consequentialism. © Oxford University Press 2020.