When composing an answer, defendants may assert their own claims against the plaintiff. For all purposes within the trial, the plaintiff acts in a defensive posture regarding these counterclaims, and the defendant acts in an offensive posture. Thus, for example, the defendant bears the burden of proof on counterclaims.
There are both permissive and compulsory counterclaims. "Common law compulsory counterclaims" are counterclaims that, if successful, would nullify the plaintiff's claim. If defendants do not raise these counterclaims, they cannot sue on them later in a different lawsuit. For example, if Company A sues Company B for breach of contract, and Company B did not make any counterclaims, Company B could not later sue Company A for fraudulently inducing it to sign the contract in the first place. See Res Judicata. See, e.g., Cardinal Chemical v. Morton International.
Many jurisdictions have also created additional classes of compulsory counterclaims. For example, Rule 13 of the Federal Rules of Civil Procedure requires defendants to raise as a counterclaim any claim they have against parties already in the lawsuit if the claim "arises out of the same transaction or occurrence" as one of the plaintiff's claims.
Permissive counterclaims are counterclaims addressing matters unrelated to the plaintiff's claims. This allows parties to settle all of their otherwise unrelated disputes in one single lawsuit.
If the defendants' counterclaims address the same basic issues as the plaintiff's claims, courts usually address the claims and counterclaims at the same time. If the counterclaims invovle distinctly different issues or facts, the court may choose to address them separately.
See Civil Procedure.