By the guarantee on first demand contract, also called guarantee on first demand, generally unilateral and onerous, the guarantor (guarantor), who is a professional (bank, savings bank, credit institution or insurance company) guarantees the benefit (debt), or the appearance of debt, of his principal, obligating himself to pay the beneficiary of the guarantee an amount of money, as compensation, when the beneficiary notifies the guarantor, formally, independently of the previously agreed event normally linked to obtaining a certain benefit, or a certain economic result derived from their legal relationship described in the guarantee. That is, it is an independent guarantee unrelated to the guaranteed contract, which deploys its effects by simple notification of the debtor's default, without requiring demonstration that this has actually occurred.

The request for payment of the guarantee can be made by the beneficiary objectively or independently of the guaranteed business, so that the guarantor will be forced to pay without questioning anything. It will then be the guarantor who demands responsibility from the beneficiary who has abused the guarantor by demanding payment without the de facto situation compromised by the guarantee occurring.

The "guarantee at first request" or, as Italian doctrine calls it, "autonomous guarantee contract", "pure guarantee contract" or "unconditional", is a legal institution, a contractual figure, of exclusively doctrinal construction, which historically emerged in German doctrine, as a guarantee intended to confer legal security to foreign trade. In Germany then all this gave rise to contractual clauses called garantieverträg, or autonomous guarantee contract, in which the accessory nature of said guarantee is completely excluded. In Spain, it was accepted by jurisprudence, for the first time, in a ruling of the Supreme Court dated November 14, 1989.

For this area of ​​foreign trade, the International Chamber of Commerce published Uniform Rules on Contractual Guarantees, which also refer to first demand guarantees.