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Numbers : The actions of the defendant have to affect so many people that it is more practical for a few representative plaintiffs to address the claims than for all the individual plaintiffs to join together in a regular lawsuit. If twenty people are swindled in a business venture, they can file their own lawsuits. But if hundreds or thousands have been harmed, a class action allows a few to represent the multitude.
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Common claims : The claims must raise similar questions of law or fact so that it saves time to lump everybody together. If a hundred people all bought cars with defective seatbelts from the same company, a class action may be the way to handle the complaint.
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Typical cases : The people bringing the lawsuit on behalf of others (the "named plaintiffs") must have the same claims and defenses as those they are representing. For example, a person who claims his seatbelt doesn't work cannot represent those who think the seatbelt works fine but the airbags inflate dangerously.
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Fair and adequate representation : The plaintiffs and the class lawyers must be good caretakers for other people's claims. A lawyer who has made major mistakes in other class actions or a representative plaintiff who has lied under oath may not meet this criterion.
If these five criteria are met and all the class wants is an order telling the defendant to stop doing something - dumping waste into a river, for example - a judge will usually agree to "certify" the class. (Certification allows a few designated appointees to represent the other members of the class.) If the class also wants money, they have to prove that the claims and injuries are so similar that they can be well managed in one lawsuit. Mass tort cases, in which a lot of people have been physically injured in a disaster or by a dangerous product, are often denied class action status because the causes and extent of individual injuries vary so much from person to person.
