In recent decades, different manifestations of Social and Complementary Currencies (SCCs) have emerged. SCCs are exchange mechanisms that offer an alernative to legal tender, aiming to stimulate trade within a circle. Their use is justified by the very definition of money as an agreement or law made by the community. Such currencies can be divided into six categories: currencies backed by official currencies to optimize the circulation of the legal tender by retaining it; currencies backed by other goods and/or services to inject liquidity into the community; currencies issued by the public authority that are circulated extensively because of their usefulness for paying taxes; mutual credit systems where members' positive or negative balances equate to the right to ask for the equivalent value of goods and/or services or the duty to provide them, respectively; SCCs issued as bank credit, with counter-cyclical effects to stabilize economic activities; and Fiat SCCs, which come into being without collateral, and need to be carefully managed to avoid accumulations in some businesses or overissue leading to hyperinflation. Each model's advantages and disadvantages must be studied carefully to decide which is most appropriate.