ontract law
Main article: Contract

The famous Carbolic Smoke Ball advertisement to cure influenza was held to be a unilateral contract
Contract law concerns enforceable promises, and can be summed up in the Latin phrase pacta sunt servanda (agreements must be kept).[194] In common law jurisdictions, three key elements to the creation of a contract are necessary: offer and acceptance, consideration and the intention to create legal relations. In Carlill v Carbolic Smoke Ball Company a medical firm advertised that its new wonder drug, the smokeball, would cure people's flu, and if it did not, the buyers would get £100. Many people sued for their £100 when the drug did not work. Fearing bankruptcy, Carbolic argued the advert was not to be taken as a serious, legally binding offer. It was an invitation to treat, mere puffery, a gimmick. But the Court of Appeal held that to a reasonable man Carbolic had made a serious offer, accentuated by their reassuring statement, "£1000 is deposited". Equally, people had given good consideration for the offer by going to the "distinct inconvenience" of using a faulty product. "Read the advertisement how you will, and twist it about as you will", said Lord Justice Lindley, "here is a distinct promise expressed in language which is perfectly unmistakable".[195]

"Consideration" indicates the fact that all parties to a contract have exchanged something of value. Some common law systems, including Australia, are moving away from the idea of consideration as a requirement. The idea of estoppel or culpa in contrahendo, can be used to create obligations during pre-contractual negotiations.[196]

Civil law jurisdictions treat contracts differently in a number of respects, with a more interventionist role for the state in both the formation and enforcement of contracts.[197] Compared to common law jurisdictions, civil law systems incorporate more mandatory terms into contracts, allow greater latitude for courts to interpret and revise contract terms and impose a stronger duty of good faith, but are also more likely to enforce penalty clauses and specific performance of contracts.[197] They also do not require consideration for a contract to be binding.[198] In France, an ordinary contract is said to form simply on the basis of a "meeting of the minds" or a "concurrence of wills". Germany has a special approach to contracts, which ties into property law. Their 'abstraction principle' (Abstraktionsprinzip) means that the personal obligation of contract forms separately from the title of property being conferred. When contracts are invalidated for some reason (e.g. a car buyer is so drunk that he lacks legal capacity to contract)[199] the contractual obligation to pay can be invalidated separately from the proprietary title of the car. Unjust enrichment law, rather than contract law, is then used to restore title to the rightful owner.[200]

Torts and delicts