In contract law, impossibility is an excuse for the nonperformance of duties under a contract, based on a change in circumstances (or the discovery of preexisting circumstances), the nonoccurrence of which was an underlying assumption of the contract, that makes performance of the contract literally impossible.

The doctrine of impracticability in the common law of contracts excuses performance of a duty, where the said duty has become unfeasibly difficult or expensive for the party who was to perform.

Impracticability is similar in some respects to the doctrine of impossibility because it is triggered by the occurrence of a condition which prevents one party from fulfilling the contract. The major difference between the two doctrines is that while impossibility excuses performance where the contractual duty cannot physically be performed, the doctrine of impracticability comes into play where performance is still physically possible, but would be extremely burdensome for the party whose performance is due. Thus, impossibility is an objective condition, whereas impracticability is a subjective condition for a court to determine.

Typically, the test U.S. courts use for impracticability is as follows (with a few variations among different jurisdictions):

1. There must be an occurrence of a condition, the nonoccurrence of which was a basic assumption of the contract,

2. The occurrence must make performance extremely expensive or difficult

3. This difficulty was not anticipated by the parties to the contract (note: some jurisdictions require that there be no measure within the contract itself to allocate risk between the parties)

An illegal agreement under the common law of contract, is one that the court will not enforce because the purpose of the agreement is to achieve an illegal end. The illegal end must result from performance of the contract itself. The classic example of such an agreement is a contract for murder.

Unclean hands doctrine, or dirty hands doctrine, is an equitable defense in which the defendant argues that the plaintiff is not entitled to obtain an equitable remedy because the plaintiff is acting unethically or has acted in bad faith with respect to the subject of the complaint—that is, with "unclean hands". The defendant has the burden of proof to show the plaintiff is not acting in good faith. The doctrine is often stated as "those seeking equity must do equity" or "equity must come with clean hands". This is a matter of protocol, characterized by A. P. Herbert in Uncommon Law by his fictional Judge Mildew saying (as Herbert says, "less elegantly"), "A dirty dog will not have justice by the court".

Accord and satisfaction is a contract law concept about the purchase of the release from a debt obligation. It is one of the methods by which parties to a contract may terminate their agreement. The release is completed by the transfer of valuable consideration that must not be the actual performance of the obligation itself. The accord is the agreement to discharge the obligation and the satisfaction is the legal "consideration" which binds the parties to the agreement. A valid accord does not discharge the prior contract; instead it suspends the right to enforce it in accordance with the terms of the accord contract, in which satisfaction, or performance of the contract will discharge both contracts (the original and the accord). If the creditor breaches the accord, then the debtor will be able to bring up the existence of the accord in order to enjoin any action against him.

If a person is sued over an alleged debt, that person bears the burden of proving the affirmative defense of accord and satisfaction.