For source material on Roman Law see The Civil Law, including The Twelve Tables, The Institutes of Gaius, The Rules of Ulpian, The Opinions of Paulus, The Enactments of Justinian, and The Constitutions of Leo by SP Scott (1932). See also Obligationes (George Long), Obligationes in Smith’s Dictionary of Greek and Roman Antiquities, and Annotated Justinian Code by Fred H Blume, edited by Timothy Kearley.

Modern Civil codes have developed this material in very different ways. A useful selection of material is available at the Ius Commune Casebook site. Here are some provisions of modern codes and relevant articles (in English where possible). One useful translation engine is here.

• Brazilian Civil Code (Código Civil) - see Pagamento indevido e enriquecimento sem causa by Jadson Dias Correia. See also Do pagamento indevido e sua repetição by Ercias Rodrigues de Sousa (2002) (and see here); Enriquecimento injustificado by Renato Ribeiro; Enriquecimento sem causa by Cléya Aparecida Henz; O Enriquecimento Sem Causa No Novo Código Civil Brasileiro by Luís Manuel Teles de Menezes Leitão ; Enriquecimento sem causa no direito civil brasileiro contemporâneo e recomposição patrimonial by Maria Candida do Amaral Kroetz (2005)

• French Civil Code (Code Civil). For an explanatory article (in French) see Les quasi-contrats. The entire code is available here et (en Français) ici. Quasi-contract is treated in article 1370ff.

• German Civil Code (Bürgerliches Gesetzbuch): Selected provisions on Restitution, courtesy of the German Law Archive, which also hosts a discussion forum on various topics, including Restitution.

• Greek Civil Code (): we have a commentary () on the main provisions, contributed by Maria Canellopoulou Botti. The main code provisions are:
  

  • Unjust enrichment (Book 2, Chapter XXXVIII) (aussi en Francais)
  • Gestion d'affaires (Book 2, Chapter XXIII)

• Louisiana Civil Code (Code Civil). See The Former Provisions of the Louisiana Civil Code on Obligations and Contracts

• Mexican Civil Code (Codigo Civil):

  • De la gestión de negocios (Libro 4, Titulo Primero, Capitulo IV)
  • De la subrogacion (Libro 4, Titulo Tercero, Capitulo III)

   

• Philippines Civil Code: Title XVII arts 2142-2175: Quasi-contracts

• Portuguese Civil Code (Código Civil):     

  • Gestão de negócios (Livro 2, título 1, secção 3)

  • Enriquecimento sem causa (Livro 2, título 1, secção 4)

• (Proposed) Principles Of European Contract Law (2002). See especially Article 15:104 (Illegality – Restitution).

• Québec Civil Code (Code Civil): §1482ff, De la gestion des affaires; §1491ff, De la réception de l'indu; §1493ff, De l'enrichissement injustifié; §1699ff, Restitution des Prestations (the entire code is available in English on CANLII)

• Russian Civil Code: Part II, Section IV, Chapter 60: Obligations Due to Unjust Enrichment

• Spanish Civil Code (Código Civil): Título XVI, Capítulo Primero, De los cuasi contratos; see also Las nulidades de los contratos (J Delgado y M^a Angeles Parra, 2003)

• Swiss Civil Code - Swiss Law - Law of Contracts by Eugen Bucher (book extract)

• Taiwan Civil Code - Arts. 179-183

• Tunisian Civil Code (Code des Obligations et des Contrats) (also here):

  • Des causes des obligations (Livre 1, titre 1, art- 0001)
  • Obligations résultant des quasi-contrats (Livre 1, titre 2, chapitre 2)
  • De la subrogation (Livre 1, titre 4, chapitre 2)