Joinder, Impleader, and Intervention: Who Ends Up in One Lawsuit (1L Civ Pro)

Joinder rules answer a single practical question: who and what can be packed into one lawsuit instead of spread across many. They govern which claims a party may bring together and which parties may — or must — be in the case. Get the framework and a wall of rule numbers turns into a short, ordered checklist.

Two ideas recur throughout. First, the "same transaction or occurrence" test, which asks whether claims or parties share a common factual core. Second, a "common question of law or fact," which keeps related disputes efficient to try together. Most joinder rules are built from those two ideas.

Rule 18 is the permissive claim-joinder rule, and it is generous: once a party has a claim against an opposing party, that party may join as many additional claims as it has against that opponent, related or not. There is no transactional requirement to stack claims under Rule 18.

The limit is jurisdictional, not procedural. You can join an unrelated claim under Rule 18, but the court still needs subject-matter jurisdiction over it. An unrelated state-law claim with no diversity may have nowhere to anchor, because supplemental jurisdiction requires a common nucleus of operative fact.

Rule 20 governs permissive party joinder — when multiple plaintiffs or defendants can be in the same suit. The test has two prongs that both must be met: the claims must arise out of the same transaction, occurrence, or series of transactions, and there must be a common question of law or fact.

So three drivers injured in the same multi-car collision can sue together as plaintiffs because their claims share the same occurrence and obvious common questions. Three drivers injured in three unrelated crashes cannot, even against the same insurer, because the transactions differ. Rule 20 is about relatedness; Rule 18 is about quantity.

Rule 19 covers parties who should be joined for the case to be resolved fairly. A party is "required" if the court cannot grant complete relief without them, or if their absence would impair their interest or expose an existing party to double or inconsistent obligations.

If a required party can be joined, the court orders it. If joining them would destroy subject-matter jurisdiction (for example, ruining complete diversity), the court asks whether the case can proceed in "equity and good conscience" without them. If it cannot, that party is indispensable and the case is dismissed. This is the old "necessary versus indispensable" analysis.

Rule 13 handles claims between parties already in the case. A counterclaim runs against an opposing party (defendant against plaintiff). A crossclaim runs against a co-party (one defendant against another) and must arise from the same transaction or occurrence.

The exam-critical split is compulsory versus permissive counterclaims. A counterclaim arising from the same transaction or occurrence as the opposing party's claim is compulsory — you must assert it in this case or lose it forever. A counterclaim from an unrelated transaction is permissive; you may raise it now or save it for a separate suit.

Impleader lets a defendant bring in a new third party who may be liable to the defendant for all or part of the plaintiff's claim — classically for indemnity or contribution. The defendant becomes a third-party plaintiff, and the new party is a third-party defendant.

The key limit: impleader is derivative. The defendant cannot implead someone simply because that person is also at fault to the plaintiff. The third party must owe the defendant — "if I lose to the plaintiff, you owe me." A contractor sued for a defect can implead the subcontractor who actually did the work because the sub would owe the contractor indemnity.

Intervention is how an outsider forces their way into an existing lawsuit. It comes in two forms. Intervention of right (Rule 24(a)) is available when the would-be intervenor has an interest in the subject of the action that may be impaired if the case proceeds without them, and no existing party adequately represents that interest.

Permissive intervention (Rule 24(b)) is discretionary: the court may allow it when the intervenor's claim or defense shares a common question of law or fact with the main action, weighing whether intervention would cause undue delay or prejudice. The difference: of right means the court must let them in; permissive means the court may.

Students routinely blur claim joinder and party joinder. Rule 18 is about how many claims one party can stack against an opponent — and it has no relatedness requirement. Rule 20 is about which parties belong together — and it demands both a shared transaction and a common question. Adding claims is easy; adding parties requires relatedness.

The second frequent slip is treating impleader as a way to point at anyone at fault. Rule 14 only reaches a third party who would be liable to the defendant. "That person also harmed the plaintiff" is not enough; the third party must owe the defendant for the defendant's exposure.