Offer and acceptance are two essential elements required to form an enforceable contract. An offer is made when one party makes a promise to do or refrain from doing some specified act in the future. And an acceptance occurs when an offer is accepted unequivocally—without conditions or ambiguity.

In other words, the acceptance must mirror the terms of the offer. If the acceptance varies from the terms of the offer it is considered a rejection of the offer and a counteroffer.

Acceptance may be expressed through words, deeds, or the performance called for in the contract.

In Illinois, as in other states, the formation of a contract requires an offer, acceptance, and consideration. An offer is a clear proposal made by one party (the offeror) to another (the offeree), indicating a willingness to enter into a contract on certain terms. Acceptance, on the other hand, must be an unequivocal agreement to the terms of the offer without modification; it must be a 'mirror image' of the offer. If the acceptance alters the terms or adds new ones, it is not a true acceptance but rather a counteroffer, which the original offeror can then accept or reject. Acceptance can be communicated through words, written or spoken, or through actions that clearly indicate agreement to the contractual terms, such as beginning the performance of the contract. Under Illinois law, the acceptance must be communicated to the offeror unless the offeror has waived the need for communication. The timing of acceptance is also important, as an offer can be withdrawn before it is accepted, and any acceptance after withdrawal is ineffective.