Under the American Bar Association’s Model Rules, “A fee may be contingent on the outcome of the matter for which the service is rendered.” The lawyer is paid a percentage of what the client recovers. If nothing is recovered, the lawyer receives no fee.
The terms must be fixed in advance. The agreement must be “in a writing signed by the client” and must state the percentages for “settlement, trial or appeal.” At the end, the lawyer gives the client “a written statement stating the outcome of the matter.”
The client may still owe the expenses of a lost case. The agreement must say so clearly.