Champerty is the maintenance of a person in a lawsuit on condition that the subject matter of the action is to be shared with the maintainer. Among laypersons, this is known as "buying into someone else's lawsuit."
At common law, maintenance and champerty were both crimes and torts, as was barratry, the bringing of vexatious litigation. This is generally no longer so as during the nineteenth century, the development of legal ethics tended to obviate the risks to the public, particularly after the scandal of the Swynfen will case (1856-1864). However, the principles are relevant to modern contingent fee agreements between a lawyer and a client and to the assignment by a plaintiff of his rights in a lawsuit to someone with no connection to the case. Champertous contracts can still, depending on jurisdiction, be void for public policy or attract liability for costs.
Proponents contend that champerty allows indigent plaintiffs access to expensive and expansive legal services they might not otherwise be able to afford. Opponents note that the ubiquity of attorneys willing to take cases "on contingency" obviates the need for champerty by non-attorneys. The opponents further note that the ethical codes which limit the percentage of recovery which an attorney may charge on contingency are not applicable to non-attorneys, thereby exposing litigants to potentially predatory practices.
It is also useful to note that what may generally be categorized as champerty may not be so clear. At one end of the spectrum would be the assignment of a cause of action relating to a personal tort. The accident example above arguably only narrowly avoids being clearly champertous in that an evaluation "service" is being performed.
On the other end, the assignment of a cause of action which is subsidiary, yet integrally related to a bona fide transfer of a property or other right should not be champertous. As an example, the assignment of a patent may include the transfer of a right to sue for past damages. In this case, certain jurisdictions recognize that the assignment of all rights associated with a species of intellectual property may for practical purposes include the right to sue. Therefore, these transactions will not constitute champerty.