(Latin: “he has undertaken”), in common law, an action to recover damages for breach of contract. Originating in the 14th century as a form of recovery for the negligent performance of an undertaking, this action gradually came to cover the many kinds of agreement called for by an expanding commerce and technology.

The concept of assumpsit was first introduced in cases in which the defendant damaged goods entrusted to him by the plaintiff—e.g., where the defendant had taken the plaintiff's horse in order to transport it across a river and negligently caused the ferry to overturn so that the horse drowned.

Assumpsit did not become a contractual remedy in the modern sense until two modifications occurred: (1) the emphasis shifted from the negligent act of the defendant to the defendant's failure to keep his promise; and (2) the action was made available as a remedy in situations where the defendant did something improperly or neglected to do something he had promised to do.