Tax defiers--or "tax protesters" as they've traditionally been known--glom onto one kooky, discredited theory or another as to why the income tax is illegal or doesn't apply to them personally or doesn't cover their normal sources of income. (Example: Only foreign income, or only earnings of federal employees are taxable.) They typically file returns showing zero income or simply stop filing.

FORBES: Snipes' Sentence A Big Win For Tax Officials

The World Trade Organization has ruled that our tax export subsidies--known as Foreign Sales Corporation and Extraterritorial Income Exclusion provisions--are illegal.

FORBES: Fact and Comment

Others, such as groups in the Middle East, received income indirectly from sympathisers involved in the illegal trade.

BBC: Fake goods linked to terrorism

Taking them now, and the income that flows from them, is illegal.

BBC: Sean Quinn's jailing reminiscent of Greek tragedy

While owning foreign real-estate isn't illegal in Russia, legislators are required by law to declare their property and income.

WSJ: Russian Lawmaker Exits Amid Florida Condo Storm

The Senate inquisitors aren't accusing the tech companies of doing anything illegal but of cleverly exploiting loopholes in U.S. tax laws that allow them to shelter income overseas.

CNN: Don't blame Apple; blame the tax code

The contracts with pensioners that were supposed to fund that steady stream of income for investors are, according to legal experts and a growing number of judges, illegal and unenforceable.

FORBES: Magazine Article

Nevertheless, illegal immigration from Mexico may lower wages, or at least slow the growth in wages, for low-income workers, especially minorities and those without high school degrees.

FORBES: The GOP And Immigration: Good Economics, But What About The Politics?