Contract law and tort law are the two main pillars of the law of obligations. However, there are other forms of obligations as well, in particular those stemming from unjustified enrichment. Consequently, the question arises as to which principles a European law of enrichment could be based upon. The law of unjustified enrichment is a field whose modern continental European conception rests upon the generalisation and development of principles of Roman law; notwithstanding individual differences, a core unity has remained to a great extent. A more recent development has been the series of events delivering English law in this regard from its pronounced isolation and re-establishing its contact with the continental European tradition. To this end, a comparative symposium at the University of Cambridge was organised by Reinhard Zimmermann during his stay at Cambridge, in cooperation with David Johnston. Therein, eleven questions decisive for the structure of the law of unjustified enrichment were addressed by a pair of scholars; each question was examined by a scholar with a common law background as well as by a jurist with a civil law background. The paradigmatic foreground of the discussion was formed by German and English law (as leading representatives of the civil law and common law world) as well as the two mixed legal systems of South Africa and Scotland. A book presenting the outcome was released in spring 2002 by the Cambridge University Press: David Johnston/Reinhard Zimmermann (eds.), Unjustified Enrichment: Key Issues in Comparative Perspective, 2002. It contains the following chapters:
 
I. Introduction
David Johnston (Edinburgh) and Reinhard Zimmermann (Regensburg): Unjustified enrichment: surveying the landscape

II. Enrichment 'without legal ground' or unjust factor approach
Sonja Meier (Regensburg): Unjust factors and legal grounds
Thomas Krebs (London): In defence of unjust factors

III. Failure of consideration
Robin Evans-Jones (Aberdeen) and Katrin Kruse (Aberdeen): Failure of consideration
Graham Virgo (Cambridge): Failure of consideration: myth and meaning in the English law of restitution

IV. Duress and fraud
Mindy Chen-Wishart (Oxford): In defence of unjust factors: a study of rescission for duress, fraud and exploitation
Jaques du Plessis (Stellenbosch): Fraud, duress and unjustified enrichment: a civil-law perspective

V. Change of position
James Gordley (Berkeley): Restitution without enrichment? Change of position and Wegfall der Bereicherung
Phillip Hellwege (Regensburg): Unwinding mutual contracts: restitutio in integrum v. the defence of change of position

VI. Illegality
W.J. Swadling (Oxford): The role of illegality in the English law of unjust enrichment
Gerhard Dannemann (Oxford): Illegality as defence against unjust enrichment claims

VII. Encroachment and restitution for wrongs
Janet O´Sullivan (Cambridge): Reflections on the role of restitutionary damages to protect contractual expectations
Hanoch Dagan (Tel-Aviv): Encroachments: between private and public

VIII. Improvements
Andrew Kull (Atlanta): Mistaken improvements and the restitution calculus
James Wolffe (Edinburgh): Enrichment by improvements in Scots law

IX. Discharge of another person's debt
Simon Whittaker (Oxford): Performance of another's obligation: French and English law contrasted
Hector L. MacQueen (Edinburgh): Payment of another's debt

X. Third-party enrichment
Peter Birks (Oxford): 'At the expense of the claimant': direct and indirect enrichment in English law
Daniel Visser (Cape Town): Searches for silver bullets: enrichment in three-party situations

XI. Proprietary issues
George Gretton (Edinburgh): Proprietary issues
Lionel Smith (Montreal): Property, subsidiarity and unjust enrichment

XII. Taxonomy
Ewan McKendrick (Oxford): Taxonomy: does it matter?
Niall R. Whitty (Edinburgh): Rationality, nationality and the taxonomy of unjustified enrichment
 
Thematically linked to this effort was the conference of the private law section of the Deutschen Gesellschaft für Rechtsvergleichung ("German Society for Comparative Law") taking place on the 17th through the 20th of September 2003 in Dresden. Titled "Grundstrukturen eines europäischen Bereicherungsrechts", it focused on principles of a European law of unjustified enrichment.
Speakers included:
 
Christiane Wendehorst (Göttingen): Die Leistungskondiktion in rechtsvergleichender Perspektive
Thomas Krebs (London): Eingriffskondiktion und "restitution for wrongs"
Jacques du Plessis (Stellenbosch): Towards more rational structures of liability for unjustified enrichment: thoughts from two mixed jurisdictions
Hugo van Kooten (Utrecht): The structure of liability for unjustified enrichment in Dutch law, with references to French and Italian law
Mark Gergen (Texas): The structure of the proposed new Restatement on Restitution for the United States
Stephen Swann (Osnabrück): The structure of liability for unjustified enrichment in the first proposals of the Study Group on European Civil Code
 
A conference report can be found in the Juristen Zeitung (JZ 2004, p. 399 ff.). The complete conference proceedings were published in 2005 by Mohr Siebeck: Reinhard Zimmermann (ed.), Grundstrukturen eines Europäischen Bereicherungsrechts.