Despite its general informality and incorporation of flexible, open-ended rules, the CISG provides specific rules of offer and acceptance to determine whether a valid contract has been concluded. The rules of offer and acceptance, concerning the necessary content, timing, and revocation of offers, are contained in Articles 14 through 24. A valid offer must “be addressed to one or more specific persons,” be “sufficiently definite,” and indicate the offeror's intention “to be bound in case of acceptance.” If the offer is not addressed to “one or more specific persons, it is merely an invitation to offer, unless the contrary is clearly indicated by the person making the proposal. Identification of the goods, quantity, and price are the essential elements that determine whether the offer fulfills the “sufficiently definite” requirement. An offer does not fail for lack of definiteness, however, if these terms are not expressly fixed. Article 14(1) allows such terms to be “implicitly” fixed or provided for in some other way.

There are numerous, highly specific rules that control the effectiveness of offers and revocation of offers. An offer becomes effective when it reaches the offeree. Article 24 interprets “reaches” to mean that the offer has been communicated orally, delivered personally, or delivered to the offeree's place of business, mailing address, or habitual residence. If the offer is revoked before it reaches the offeree, it becomes ineffective even if the offer stated that it was irrevocable.