Assumpsit ("he has undertaken," from Latin, assumere) is a form of action at common law for the recovery of damages caused by the breach or non-performance of a simple contract, either express or implied, and whether made orally or in writing.
Assumpsit was the word always used in pleadings by the plaintiff to set forth the defendant's undertaking or promise, hence the name of the action. Claims in actions of assumpsit were ordinarily divided into (a) common or indebitatus assumpsit, brought usually on an implied promise, and (b) special assumpsit, founded on an express promise. The actual "causes of action" that could be pleaded through assumpsit were known as the "common counts," and could be pleaded in a very terse, compact style.
The Common Law Procedure Act 1852 abolished the common law forms of action in England and Wales. Furthermore, assumpsit as a form of action became obsolete in the United Kingdom after the passing of the Judicature Acts of 1873 and 1875.
In the United States, assumpsit, like the other forms of action, became obsolete in the federal courts after the adoption of the Federal Rules of Civil Procedure in 1938. Thirty-five states have moved to rules similar to the FRCP (see Civil procedure in the United States), which have replaced the various forms of action with the civil action. However, many states continue to recognize assumpsit as a common law or statutory cause of action or allow the use of the old "common counts" as causes of action. For example, California has a special "common counts" cause of action form (to be attached to an optional form complaint) based directly on the old common counts that were pleaded in assumpsit.