Today, the power to impose taxes, far from being mere discretionary plying, unmotivated and sole, is manned by a number of limitations and guarantees for the protection of the taxpayer: a particular role in this context, it is covered by the principle of ability to pay under article 53 of Constitution, as a result of which the legislator is not free to subject to tax any fact of life, being able to apply the tax only to cases that are manifestations of wealth. Therefore, the ability to pay presents itself, at the same time, as assumed, limit and measure of taxation. In the absence of a normative notion, in the recent past, the ability to pay has been depicted as a kind of “empty box”. Nevertheless, it is necessary to indicate, positive and symptomatic, individual indexes of the principle of ability to pay. Among the “direct” indices can be counted the income, assets and increases in value, while constitute “indirect indices” consumption, business or transfer of assets. The paradigm cannot be divorced from the source system and normative values, having to be coordinated to the inviolable rights of man and the duties required by political, economic and social solidarity, as a basis for the constitutional state: the duty of each person to participate in public spending becomes the mandatory principle of social solidarity, which descends directly from article 2 of Constitution. The Constitutional Court, in reconstructing the scope of the principle, has identified three requirements that must comply with the ability to pay: effectiveness, certainty and timeliness. The ability to pay is inextricably linked to the principles of reasonableness and tax equality, so that equal situations should match the same taxation regimes and, correlatively, to different situations an unequal tax treatment.