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 Oregon, 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 way by the defendant's actions. Mass torts in Oregon can be filed as class action lawsuits, which allows for the collective legal claims of many individuals to be resolved in a single court proceeding. This is particularly common in cases involving exposure to harmful substances, defective products, or large-scale invasions of privacy such as data breaches. Oregon follows the Federal Rules of Civil Procedure for class actions, which require that the case must meet certain criteria such as having common questions of law or fact, representative parties with claims typical of the class, and a class size so large that individual suits would be impractical. Once a case is certified as a class action, it can proceed to litigation or settlement negotiations on behalf of all class members.