Tax noncompliance describes a range of activities that are unfavorable to a state's tax system. These include tax avoidance, which refers to reducing taxes by legal means, and tax evasion which refers to the criminal non-payment of tax liabilities.

Groups that do not comply with taxes include tax protesters and tax resisters. Tax protesters attempt not to pay tax believing that they have discovered interpretations of the law that show that they are not subject to being taxed, whilst tax resisters refuse to pay a tax for conscientious reasons (because the resister does not want to support the government or some of its activities). Tax resisters typically do not take the position that the tax laws are themselves illegal or do not apply to them (as tax protesters do) and they are more concerned with not paying for particular government policies that they oppose. Because taxation is often perceived as onerous, governments have always struggled with tax noncompliance since the beginning of civilization.