This chapter examines the forms of ignorance that defeat and sometimes create legal liability. Although chapter focus is primarily on the treatment of ignorance in the criminal law, mention in passing the role of ignorance in torts, breaches of contract, and other civil lawsuits. It has the case of possible exculpatory ignorance because it occupies an intermediate position between ordinary factual ignorance of a nonnormative fact that the law deems legally relevant and ignorance of the content of the criminal law. In many cases, the failure of ignorance of the criminal law to excuse seems unduly harsh and even perverse. And many states have close to that many, with the result that a US citizen is subject to thousands of federal and state criminal laws. Just as ignorance can be exculpatory in criminal law, so also can it be inculpatory, as it can be in the moral realm as well.