A mass tort is a civil lawsuit that involves hundreds or thousands of plaintiffs and one or a few defendants in which the plaintiffs have the same or similar claims against the defendant or defendants. Mass tort lawsuits are often filed as class action lawsuits (a procedural mechanism for joining the plaintiffs in one lawsuit), and are typically based on alleged wrongful acts (torts) such as exposure of the plaintiffs to harmful substances (asbestos or benzene), or defective products (pharmaceuticals or medical devices), or invasions of privacy (data breaches).

In Washington State, mass torts are civil actions that involve a large number of plaintiffs against one or a few defendants. These plaintiffs generally have similar legal issues or have been harmed in a similar manner by the defendant's actions. Mass torts in Washington are often related to exposure to harmful substances, defective products, or privacy violations such as data breaches. While mass torts can be filed individually, they are frequently brought as class action lawsuits, which allows for the consolidation of numerous similar individual claims into a single action for efficiency and consistency. This procedural mechanism is governed by both state law and the Federal Rules of Civil Procedure when applicable. The Washington State court system handles these cases under the Civil Rules for Superior Court, particularly CR 23, which outlines the requirements for class action lawsuits. Additionally, federal laws may apply if the case involves federal jurisdiction, such as interstate matters or significant questions of federal law.