Tax noncompliance is a range of activities that are unfavorable to a government's tax system. This may include tax avoidance, which is tax reduction by legal means, and tax evasion which is the criminal non-payment of tax liabilities. The use of the term "noncompliance" is used differently by different authors. Its most general use describes non-compliant behaviors with respect to different institutional rules resulting in what Edgar L. Feige calls unobserved economies. Non-compliance with fiscal rules of taxation gives rise to unreported income and a tax gap that Feige estimates to be in the neighborhood of $500 billion annually for the United States.

In the United States, the use of the term 'noncompliance' often refers only to illegal misreporting. Laws known as a General Anti-Avoidance Rule (GAAR) statutes which prohibit "tax aggressive" avoidance have been passed in several developed countries including the United States (since 2010), Canada, Australia, New Zealand, South Africa, Norway and Hong Kong. In addition, judicial doctrines have accomplished the similar purpose, notably in the United States through the "business purpose" and "economic substance" doctrines established in Gregory v. Helvering. Though the specifics may vary according to jurisdiction, these rules invalidate tax avoidance which is technically legal but not for a business purpose or in violation of the spirit of the tax code. Related terms for tax avoidance include tax planning and tax sheltering.

Individuals that do not comply with tax payment include tax protesters and tax resisters. Tax protesters attempt to evade the payment of taxes using alternative interpretations of the tax law, while tax resisters refuse to pay a tax for conscientious reasons. In the United States, tax protesters believe that taxation under the Federal Reserve is unconstitutional, while tax resisters are more concerned with not paying for particular government policies that they oppose. Because taxation is often perceived as onerous, governments have struggled with tax noncompliance since the earliest of times.