Sometimes, we accept "I'm sorry, I didn't know" as an excuse, but at other times we do not. When are we justified in claiming that a person should have known better and that they are, therefore, responsible and blameworthy for their ignorant wrongdoing? Through a detailed investigation of ignorant wrongdoing, I establish conditions of responsibility for ignorant wrongdoing that not only provide a coherent justification for many of our standard judgements regarding responsibility and blame but also enable me to defend those judgements against objections and revisionist perspectives. I argue that: (1) Persons are responsible for their ignorant wrongdoing if and only if they are responsible for their ignorance. And that: (2) Persons are responsible for their ignorance if and only if: (a) They possess the rational capacity to accurately appreciate the relevant reasons. (b) There are no limitations in their circumstances that make it unreasonably difficult for them to accurately appreciate the relevant reasons. I argue that if both conditions (a) and (b) are met, then it is reasonable to claim that a person should have known better because they have had fair opportunity to avoid both their ignorance and their wrongdoing that results. In developing this account of responsibility for ignorant wrongdoing, I argue that internalist accounts of practical reasons are untenable from the perspective of responsibility and blame.