Qui tam is a Latin phrase given to the action of a plaintiff suing for the state as well as himself. The plaintiff bringing the action, if he wins, receives part of the penalty recovered. The balance of the penalty is awarded to the state. Qui tam is a legal provision under the False Claims Act that allows for a private individual (called a whistleblower) with knowledge of past or present fraud committed against the United States federal government to bring suit on its behalf.

The individual bringing the suit is also called the "relator." In qui tam cases, the relator need not have been personally harmed by the defendant's conduct. The False Claims Act grants the relator between 15-30% of any settlement amount. In addition, the statute provides for payment of the relator's attorneys' fees. Attorneys are necessary in qui tam lawsuits because the action is taken on behalf of the government and that may only be done by an attorney.

Qui tam is a very successful tool in combating fraud. This law arms private citizens with direct knowledge of fraud with a weapon to prosecute government contractors and others defrauding the government and to share in the monetary recovery.

There are several types of cases that are filed as qui tam actions. One of the most common types of cases filed is the mischarging case. Mischarging cases generally involve filing false claims for goods and services that were never provided or delivered. A very common mischarging situation is employee labor charged to a government contract not worked on. Other common schemes are claims made to the government for medical services performed by a physician when the service was actually performed by a nurse or other healthcare provider that should have been billed at a much lower rate.

Yet another type of qui tam case is the false negotiation or defective pricing that involves the submission of false cost and pricing data to the government. This takes place during the negotiation of a contract that subsequently results in an inflated contract price. Other cases involve product and service substitution such as falsely certifying that a product meets specifications and false testing schemes certifying the reliability of an unreliable product.

Filers of qui tam legal action are usually employees who have blown the whistle on employers; former employees who may have been terminated or quit under duress as a result of trying to blow the whistle internally; competitors/subcontractors who have knowledge of fraud; state and local governments who bring action against contractors and medical providers as a means of recovering state or local revenue lost as a result of the fraudulent schemes; and federal employees with knowledge of fraud.

Some of the most common defendants in qui tam actions are government contractors and subcontractors, medical providers, private universities (for their handling of federal grants and research and development money), and state and local government agencies and officials (they are recipients of large amounts of federal money). Typically, any organization or individual person who uses federal money can be charged as a defendant in a qui tam action.

Once a qui tam action is filed, the government will either intervene in the case, dismiss the case, or settle the case, and the whistleblower will be entitled to his share of the total amount recovered by the government should the case be heard.