Restitution and quasi-contracts are legal principles that address unjust enrichment and provide remedies when formal contracts are absent. These concepts restore parties to their original positions, compensate for services rendered, and enforce promises to prevent injustice. Key elements include benefits conferred, recipient knowledge, and unjust retention. Unlike traditional contracts, quasi-contracts are implied by law. Common scenarios involve mistaken payments, emergency services, and property improvements. Limitations and defenses exist, such as unclean hands and voluntary payment doctrines.