§ 428.10.Generally

Section 428.10 is the foundation of California's cross-complaint practice. It lets a party against whom a cause of action has been asserted — by complaint or cross-complaint — file its own cross-complaint back, setting out either or both of two kinds of claims.

Subdivision (a) covers claims against the very parties who sued the cross-complainant: any cause of action that party has against them. One notable limit sits inside this subdivision — it does not authorize a cross-complaint against the plaintiff in an eminent domain action brought under Title 7 (commencing with § 1230.010) of Part 3, reflecting the government's distinct posture as condemnor in that kind of proceeding.

Subdivision (b) reaches further, letting the cross-complainant bring in a claim against a person alleged to be liable on it, whether or not that person is already a party — so long as the claim either arises out of the same transaction, occurrence, or series of transactions or occurrences as the claim brought against the cross-complainant, or asserts a claim, right, or interest in the property or controversy that is the subject of that claim. That second path is what lets a defendant pull in a third party (through the third-party practice mechanics of § 428.70) or assert an interest in disputed property even against someone not previously in the case.