Ignorance of law means want of knowledge of those laws which a person has a duty to know and which everyman is presumed to know. Ignorance can be voluntary or involuntary. It is voluntary when a person might by taking reasonable pains could have acquired the necessary knowledge. For example every man can acquire knowledge of the laws which have been promulgated. Therefore neglect to become acquainted with them is voluntary ignorance. On the other hand it is involuntary when the ignorance is not of choice and it cannot be overcome by the use of any means of knowledge known to him and within his power. For example ignorance of a law which has not yet been promulgated.

Ignorantia juris non excusat or Ignorantia legis neminem excusat is a Latin maxim which means "ignorance of the law does not excuse" or "ignorance of the law excuses no one." The rationale of this maxim is that if ignorance of law was an excuse then any person charged with a criminal offense or subject of a civil suit can claim that he or she was unaware of the law in question and avoid liability. The law imputes knowledge of all laws to all persons within its jurisdiction. The doctrine assumes that the law in question has been properly published and distributed, for example, by being printed in a government gazette, made available over the internet, or printed in volumes available for sale to the public at affordable prices.

Even though general rule that ignorance of the law or a mistake of law is no defense is deeply rooted in the American legal system, case law has recognized certain exceptions to the doctrine. For example in Cheek v. United States, 498 U.S. 192, 200-201 (U.S. 1991) the court observed that the proliferation of statutes and regulations has sometimes made it difficult for the average citizen to know and comprehend the extent of the duties and obligations imposed by the tax laws. Therefore the Congress has softened the impact of the common-law presumption by making specific intent to violate the law an element of certain federal criminal tax offenses. Thus, the Court almost 60 years ago interpreted the statutory term "willfully" as used in the federal criminal tax statutes as carving out an exception to the traditional rule that ignorance of law cannot be excused. This special treatment of criminal tax offenses is largely due to the complexity of the tax laws.