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 illegal 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.

The use of the terms tax avoidance and tax evasion can vary depending on the jurisdiction. In general, "evasion" applies to illegal actions and "avoidance" to actions within the law. The term "mitigation" is also used in some jurisdictions to further distinguish actions within the original purpose of the relevant provision from those actions that are within the letter of the law, but do not achieve its purpose. All pursue the same immediate goal, minimising the amount paid.

In particular, in the American legal system, tax evasion is a criminal action disciplined by 26 US Code §7201, under which the taxpayer who fails to pay or willfully underpays his tax liability (i.e., with criminal mens rea like stated in the James v. United States) will undergo criminal penalties. On the other side of the coin, tax avoidance happens when the taxpayer tries to lessen his tax obligation using deductions and credits to maximize after-tax income. All of this is considered legal by the IRS even though it foresees civil penalties. All things considered, the main difference between tax evasion and tax avoidance is the taxpayer's guilty mind of minimization or failure to pay the tax liability.

The U.S. Internal Revenue Service provides formal definitions: "The gross tax gap is the difference between true tax liability for a given tax year and the amount that is paid on time. It is comprised of the nonfiling gap, the underreporting gap, and the underpayment (or remittance) gap. The net tax gap is the portion of the gross tax gap that will never be recovered through enforcement or other late payments."

An important way to study the tax gap is to examine the size of the tax gap in a country by analyzing the size of the underground economy and its influencing factors. The size of the underground economy is directly related to the institutional infrastructure. The institutional infrastructure of a country mainly includes the intensity of government regulation, the establishment and implementation of laws, the degree of judicial independence, the size of effective tax rates, the effective provision of public goods or services, and the effective protection of property rights. It is generally believed that the higher the level of government regulation, the greater the size of its underground economy and the greater the tax gap. And vice versa, when government over-regulation occurs, an alternative relationship exists between the size of the underground economy and the size of the official economy. Representatives of this view are Levenson, Maloney, and Johnson. They believe that higher tax rates can raise higher tax revenues, and the government can provide higher levels of public services accordingly, thereby attracting more companies and individuals out of the underground economy, resulting in a healthy balance of "high tax rates, high taxes, high public services, and small-scale underground economy", but low-tax countries, because they do not have enough income to provide high levels of public services, will form a vicious balance of "low tax rates, low taxes, low public services, and high-scale underground economy." In the above-mentioned healthy balance, the tax gap is relatively small; in the vicious equilibrium, the tax gap is relatively large.

Tax custom is different from tax avoidance or tax evasion. It does not measure the taxation behavior of individuals, but the tax attitude of individuals. The tax custom can also be considered as the moral responsibility of the individual. Making a specific contribution to society by paying taxes on the government must fulfill this responsibility. It embodies the ethical code of conduct for individuals in taxation, although it does not require the form of law. The decline or deterioration of taxation practices will reduce the moral costs of taxpayers engaging in illegal operations or underground economic activities.