In civil procedure, a party's claim is a counterclaim if the defending party has previously (in the present action) made a claim against the claiming party.

Examples of counterclaims include:

• After a bank has sued a customer for an unpaid debt
  Debt
  A debt is an obligation owed by one party to a second party, the creditor; usually this refers to assets granted by the creditor to the debtor, but the term can also be used metaphorically to cover moral obligations and other interactions not based on economic value.A debt is created when a...
  
  , the customer counterclaims (sues back) against the bank for fraud in procuring the debt. The court will sort out the different claims in one lawsuit (unless the claims are severed).
• Two cars collide. After one person sues for damage to her car and personal injuries, the defendant counterclaims for similar property damage and personal injury claims.

Counterclaims did not exist at common law

Common law

Common law is law developed by judges through decisions of courts and similar tribunals rather than through legislative statutes or executive branch action...

; they are an invention of modern civil procedure

Civil procedure

Civil procedure is the body of law that sets out the rules and standards that courts follow when adjudicating civil lawsuits...

.

Under the United States Federal Rules of Civil Procedure

In U.S. federal courts, counterclaims can arise on various occasions, including e.g.:

• an attempt by the defendant
  Defendant
  A defendant or defender is any party who is required to answer the complaint of a plaintiff or pursuer in a civil lawsuit before a court, or any party who has been formally charged or accused of violating a criminal statute...
  
   to offset or reduce the amount/implications of the plaintiff's claim;
• a different claim by the defendant against the plaintiff;
• a claim by an impleaded
  Impleader
  Impleader is a procedural device before trial in which one party joins a third party into a lawsuit because that third party is liable to an original defendant...
  
   third-party defendant against the original defendant acting as a third-party plaintiff;
• a claim by any party against another party who has made a crossclaim against him

Compulsory v. permissive

Under the FRCP

Federal Rules of Civil Procedure

The Federal Rules of Civil Procedure govern civil procedure in United States district courts. The FRCP are promulgated by the United States Supreme Court pursuant to the Rules Enabling Act, and then the United States Congress has 7 months to veto the rules promulgated or they become part of the...

, counterclaims are either compulsory or permissive.

Permissive counterclaims comprise "any claim that is not compulsory." Such claims may be brought, but no rights are waived

Waiver

A waiver is the voluntary relinquishment or surrender of some known right or privilege.While a waiver is often in writing, sometimes a person's actions can act as a waiver. An example of a written waiver is a disclaimer, which becomes a waiver when accepted...

 if they are not. Courts rarely give permissive counterclaims the necessary supplemental jurisdiction

Supplemental jurisdiction

Supplemental jurisdiction is the authority of United States federal courts to hear additional claims substantially related to the original claim even though the court would lack the subject-matter jurisdiction to hear the additional claims independently. is a codification of the Supreme Court's...

 to be brought.

A claim is a compulsory counterclaim if, at the time of serving the pleading,

1. the counterclaim "arises out of the transaction or occurrence that is the subject matter of the opposing party's claim,"
2. AND the counterclaim "does not require adding another party over whom the court cannot acquire jurisdiction,"
3. AND "when the action was commenced, the [otherwise mandatory counterclaim] was [not] the subject of another pending action,"
4. AND

• EITHER the opposing party sued on its claim by a process that established personal jurisdiction over the pleader on that claim, (i.e., NOT by a process such as attachment
  Attachment (law)
  Attachment is a legal process by which a court of law, at the request of a creditor, designates specific property owned by the debtor to be transferred to the creditor, or sold for the benefit of the creditor. A wide variety of legal mechanisms are employed by debtors to prevent the attachment of...
  
  )
• OR (if personal jurisdiction was not established over the pleader), the pleader asserts some other mandatory counterclaim.

This last (fourth) requirement is explained in the official notes as follows:
If the counterclaim is compulsory, it must be brought in the current action or it is waived and lost forever.

Various tests have been proposed for when a counterclaim arises from the same transaction or occurrence, including same issues of fact and law, use of the same evidence, and logical relation between the claims.