Offer and acceptance are generally recognized as essential requirements for the formation of a contract, and analysis of their operation is a traditional approach in contract law. The offer and acceptance formula, developed in the 19th century, identifies a moment of formation when the parties are of one mind. This classical approach to contract formation has been modified by developments in the law of estoppel, misleading conduct, misrepresentation, unjust enrichment, and power of acceptance.

Offer.

Treitel defines an offer as "an expression of willingness to contract on certain terms, made with the intention that it shall become binding as soon as it is accepted by the person to whom it is addressed", the "offeree". An offer is a statement of the terms on which the offeror is willing to be bound. It is the present contractual intent to be bound by a contract with definite and certain terms communicated to the offeree.

The expression of an offer may take different forms and which form is acceptable varies by jurisdiction. Offers may be presented in a letter, newspaper advertisement, fax, email verbally or even conduct, as long as it communicates the basis on which the offeror is prepared to contract.

Whether the two parties have reached agreement on the terms or whether a valid offer has been made is an issue which is determined by the applicable law. In certain jurisdictions, courts use criteria known as 'the objective test' which was explained in the leading English case of Smith v Hughes. In Smith v Hughes, the court emphasized that the important thing in determining whether there has been a valid offer is not the party's own (subjective) intentions, but how a reasonable person would view the situation. The objective test is largely superseded in the UK since the introduction of the Brussels Regime in combination with the Rome I Regulation.

An offer can only be the basis of a binding contract if it contains the key terms of the contract. For example, as a minimum requirement for sale of goods contracts, a valid offer must include at least the following four terms: Delivery date, price, terms of payment that includes the date of payment and detailed description of the item on offer including a fair description of the condition or type of service. Unless the minimum requirements are met, an offer of sale is not classified by the courts as a legal offer but is instead seen as an advertisement. Under Dutch law an advertisement is in most cases an invitation to make an offer, rather than an offer.