Every civil lawsuit involves at least two parties—a plaintiff making a claim and a defendant resisting it. Beyond this basic requirement, legal systems differ slightly in their approach to the question of whether other parties may or must be joined.

In order to participate in a lawsuit as a plaintiff or as a defendant, a party must have the capacity to sue and must be a “proper” party (i.e., have standing before the court). Adult individuals have legal capacity to sue or be sued unless they are mentally incapacitated. Corporations and even groups of individuals without formal corporate status generally may assert rights in court and are liable to suit by others. The law obliges certain persons to act through another person. These persons, such as minors and those determined to be mentally incompetent, usually must act through parents or guardians. Corporations can frequently sue in their own name, though some countries (such as Sweden) require that actions involving a corporate entity be brought by or against its board of directors or a similar body.

All legal systems impose a requirement that a plaintiff have an interest for which he seeks protection in the lawsuit. Generally, only persons who have suffered an injury that can be remedied by the outcome of the lawsuit may sue—a doctrine sometimes called “standing” to sue. Furthermore, only a person who owns (or claims to own) the right or obligation under suit can be a party to a suit involving that right. In the United States this rule is frequently called the real party in interest rule, and similar rules exist in other countries (e.g., Italy and France). The real party in interest ordinarily will be the person who will ultimately benefit from any recovery obtained. In matters of public law, the ability to sue is sometimes restricted less narrowly than in pure private-law actions. For example, in France and in some U.S. states, citizens can bring actions in court to attack certain governmental expenditures.

A person ordinarily decides for himself whether he wants to attempt to enforce his rights by legal proceedings. Under some circumstances, however, his refusal to do so may cause harm to others. To address this situation, the laws of many countries authorize creditors, for example, to prosecute actions of their debtors if the debtors fail to do so.

Legal controversies are not necessarily limited to two persons—one plaintiff and one defendant. For example, in actions involving co-ownership or joint obligations, the rights of several parties may be so intertwined that it is impossible to adjudicate the rights of one person standing alone. In such circumstances, the procedural rules of many countries require that all such persons be made parties to the lawsuit. In other cases the presence of several individuals may be merely useful, but not absolutely essential, to a resolution of a dispute. In such cases the law simply “permits” the individuals to join, or be brought into, the lawsuit. It is also possible that persons not originally participating in a lawsuit may find that a suit directly or indirectly affects their interests. To avoid a multiplicity of actions, the law may authorize such persons to intervene in the pending lawsuit if their own claim has a sufficiently close connection in law or fact. Furthermore, in certain cases a defendant may bring third parties into an action, through a device called impleader, when, for example, these parties are or may be liable to the defendant on account of the claim asserted against him.

Ordinarily, a judgment binds only the parties to the lawsuit. In some situations, however, a large group of persons may stand in the same legal situation, as when they are subject to the same governmental or corporate practice or charge. Each such individual could in theory bring a separate action, but individual actions would be inefficient (and uneconomic if each individual injury was small) and might result in contradictory rulings. To overcome these obstacles, the law in the United States authorizes class actions, in which a limited number of persons sue to vindicate the rights of a much larger group; all members of this class of persons may be bound by the outcome of the suit if the active members adequately represent the absentees. Class actions have been used to challenge racial segregation, to redress overcharges and other unlawful pricing practices, and, more controversially, to seek redress in instances of widespread personal injury. Countries with a civil-law tradition generally do not authorize class actions, though in some limited situations proceedings brought by one person may affect the rights of other persons who are not party to the suit (e.g., sometimes associations, such as consumers’ groups, are authorized to sue).

In civil-law countries a person wishing to support the claim of some other party must proceed by way of direct intervention. In the United States an individual who wants to promote the claim of some other party may seek to present the court with an amicus curiae (Latin: “friend of the court”) brief, which will contain arguments in favour of the person the individual supports.

A person’s capacity to sue or be sued ordinarily is not affected if the person is an alien or nonresident, unless a state of war exists between his home country and the country in which he wishes to sue. Even a state of war generally does not destroy the capacity to be sued, though an alien may experience some disadvantages. For example, many countries withhold legal aid from aliens, particularly if the alien’s home country does not grant reciprocity. More important, many European and Latin American countries require alien plaintiffs to post security to guarantee that they will be able to reimburse the defendant for the expenses of the lawsuit, and sometimes even for additional damages, should the alien lose the case. As a result of the 1954 Hague Convention on Civil Procedure and numerous other treaties, this requirement has been eliminated between many countries. In the United States and in most other countries with a common-law tradition, the nationality of a party is not material to the issue of whether security for costs is due.