Assumpsit. In the English Courts of Common Law, a plaintiff might, in old time, fail to obtain justice if he could not bring his claim within one of the customary forms of action. The clerks in Chancery were therefore empowered by statute in 1285 to frame new writs 'on the case,' where the cause of action was in the nature of trespass. One of the new forms devised by the clerks was the action of assumpsit, wherein the plaintiff asserted that the defendant undertook (assumpsit) to do a certain act, and failed to fulfil his promise. Though in form an action for trespass, assumpsit was really an action arising out of contract; and it was extensively used in commercial and other cases. The Common Law Procedure Act, 1852, did away with the old forms of writ, and under the Judicature Acts all actions are now commenced by a writ of summons.
In the United States, assumpsit is the most common form of action. It is defined to be an action for the recovery of damages for the breach of a parol or simple contract; but in reality it is the form of action generally used for the recovery of any debt, or for the recovery of money in the hands of the defendant which in justice and equity belongs to the plaintiff. The essential is that there should be a privity of contract between the parties, either express or implied. The plaintiff may neglect the contract or waive a tort, and sue in general assumpsit, but generally a special agreement implies a special assumpsit; in the latter case, however, the promise must be proved. Assumpsit has in most of the older states by statute superseded all the common law forms of action. It is based upon the general equitable idea that what one ought to do he has assumed or promised or undertaken to do, and being equitable in its nature, takes the place of all other civil actions in those states in which the difference between equity and law has been abolished in practice, and but one form of civil action is recognised by statute.