A bilateral contract arises from the exchange of mutual, reciprocal promises between two persons that requires the performance or non-performance of some act by both parties. The promise made by one party constitutes sufficient consideration for the promise made by the other party. A unilateral contract involves a promise made by only one party in exchange for the performance or non-performance of an act by the other party. Stated differently, acceptance of an offer to form a unilateral contract cannot be achieved by making a return promise, but only by performance or non-performance of some particular act. Accordingly, acceptance of an offer to enter a unilateral contract can be revoked until performance is complete or until the date has passed for nonperformance.
It should be remembered, however, that courts are asked to interpret contracts long after they have been formed. As a result, courts will often take into account how the parties actually acted on the terms of a particular contract. Not surprisingly, courts will avoid interpreting a contract as unilateral or bilateral when such an interpretation would leave one party in the lurch or the opposite interpretation would yield a more commercially reasonable result. This is not to say that courts do not enforce one-sided agreements, but only that the evidence of the parties’ understanding must be clear before a court will do so.