To what extent can an agent be held responsible for what he does? According to Aristotle, we are answerable for our voluntary actions, the “voluntary” being “[1] that of which the origin is in oneself, [2] when one knows the particular factors that constitute the location of action.” This question, which was of paramount importance for Anscombe, led her to focus on the second, epistemic condition of responsibility. This chapter suggests that in fact, a large part of her philosophy of action can be read as an effort to build a complex and subtle answer to the question what kind of knowledge an agent must have of his own action in order for him to be accountable for it (and/or for all or some of its consequences). In Intention, Anscombe shows that for his action to be intentional under some description, an agent must have a practical knowledge of it under this very description; that is why ignorance can sometimes be put forward as a ground for exoneration. But ignorance can also be blameworthy, when the agent could and should have known what he is ignorant of. Elucidating the possibility of voluntary ignorance brings into light a fundamental obligation to know which takes its meaning from, and has its roots in, Anscombe’s Thomist background.