Introduction -- Justifying grounds : statutes, judgments, and natural obligations -- Justifying grounds : contracts -- Enrichment : general principles -- Enrichment : types of benefit -- At the claimant's expense : personal claims -- At the claimant's expense : proprietary claims -- Lack of consent and want of authority -- Mistake -- Duress -- Undue influence and unconscionable bargains -- Failure of basis : general principles -- Failure of basis : bases of transfer -- Failure of basis : deposits -- Frustrated contracts -- Anticipated contracts that do not materialise -- Free acceptance -- Necessity -- Secondary liability : overview -- Secondary liability : contribution and reimbursement -- Secondary liability : insurer's subrogation rights -- Money paid as taxes and other levies that are not due -- Ultra vires payments by public bodies -- Legal incapacity -- Illegality -- Benefits conferred under judgments and orders that are later reversed -- Change of position -- Ministerial receipt -- Bona fide purchase and good consideration -- Estoppel -- Counter-restitution impossible -- Passing on -- Limitation -- Legal incapacity -- Illegality -- Personal remedies -- Proprietary remedies : general principles -- Proprietary remedies : trusts and liens -- Proprietary remedies : subrogation to extinguished proprietary rights -- Proprietary remedies : rescission and rectification.