A class-action lawsuit can be filed by many plaintiffs against a single defendant or multiple defendants. Rules allow for this type of action because it is more efficient than forcing each plaintiff to sue individually. Under Rule 23 of the Federal Rules of Civil Procedure, there are four prerequisites for class actions to be appropriate. (1) the class must be so numerous that joinder of all individual members is impractical (a requirement known as “numerosity”); (2) there are common questions of law or fact to the class (called “commonality”) (3) the class representative bringing the lawsuit has claims that are typical of the class members (“typicality”), and (4) the representative is likely to be able to fairly and adequately protect the interests of the class.
Once the trial judge believes that the class action is appropriate, she can certify the class and the case can go forward.
Members of the class have the option of “opting out” and bringing their own lawsuits, in which case they do not share in any verdict or settlement earned by the class. If they fail to do so, they will share in any benefits awarded to the class but cannot bring separate lawsuits.