A flat tax rate is a familiar election-campaign slogan these days. Politicians calling for the introduction of a flat tax system quote examples of countries that have already implemented such systems. Unfortunately, these examples do not always show the whole picture. In the article, the author describes how the flat tax system is constructed in individual countries. It turns out that no national tax system actually meets the definition of a true flat rate. Everywhere there are tax breaks and deductions. In most countries, there is an actual progression of personal income tax (PIT) and the flat rate is only theoretical. Corporate income tax (CIT), though usually simpler than PIT, does not meet the requirements of linearity, either. In the theoretical part of the article, the author tries to explain that the dispute over the flat tax rate is ideological and-due to the absence of viable arguments-there are slim chances for a compromise in this area.