According to a famous maxim, ignorance or mistake of law is no excuse. This maxim is supposed to represent both the standard and the proper rule of law. In fact, this maxim should be qualified in both respects: ignorance and mistake of law sometimes are, and (perhaps even more often) should be, excused. But this dual qualification only reinforces the fundamental and ubiquitous assumption which underlies the discussions of the subject, namely, that the only ground of exculpation relevant to ignorance (...) or mistake of law is excuse due to lack of (sufficient) culpability. The paper challenges this consensus. I argue that, according to the best conception of justification, ignorance and mistake, including ignorance and mistake of law, could be justified. Generally, ignorance and mistake are justified when based on a rational analysis of the information the agent has and should have. The conceptual possibility of justified ignorance or mistake is demonstrated mainly with respect to ignorance or mistake of law due to reliance on the guidance of public officials or private lawyers. (shrink)