Fei USD is a decentralised stablecoin that aims to maintain a one-to-one peg with the US dollar, primarily introduced to address the inefficiencies and centralisation challenges of traditional stablecoins. Launched by Fei Labs as a part of the Fei Protocol, it employs a novel mechanism called Protocol Controlled Value (PCV) to ensure liquidity and stability. Unlike traditional stablecoins backed by fiat reserves, Fei USD utilises a decentralised reserve management system, with its underlying asset being managed within decentralised financial protocols. This method allows it to provide scalable and capital-efficient alternatives, while also introducing innovations such as direct incentives for maintaining the peg. Fei USD is deeply integrated into the DeFi ecosystem, facilitating seamless transactions and liquidity provision without relying on potentially centralised custodial entities.