Ammidon v. Smith et al. was a case heard by the United States Supreme Court in 1816 that centered around a dispute between two parties over an unpaid debt and the right to collect it from third-party individuals who had received money from the debtor. The plaintiff, Ammidon, argued that he should be able to recover his debt from those third-parties because they were aware of its origin when they accepted payment for goods or services rendered on behalf of the debtor. The defendants countered that since their transactions with the debtor did not involve any knowledge of Ammidon's claim against them, they could not be held liable for repayment of his debt without being given prior notice and opportunity to contest it in court first. In ruling on this case, Chief Justice John Marshall found in favor of Ammidon and established precedent stating that creditors have a legal right to pursue recovery against third-parties who are holding funds belonging to their debtors if such persons had knowledge at time transaction took place regarding source or origin of those funds owed by said debtor(s).

In Ammidon v. Smith et al., the Supreme Court was asked to decide whether a contract between two parties, made in Massachusetts and then performed in New Hampshire, should be governed by the law of Massachusetts or that of New Hampshire. The majority opinion held that contracts are generally subject to the laws of the state where they were formed, regardless of where performance takes place. However, Justice Story dissented from this decision on several grounds. He argued that since both states had an interest in regulating such matters within their own borders, it would be more appropriate for them to agree upon which state's laws applied rather than allowing one party unilaterally determine which set of rules would govern their agreement. Furthermore he noted that if courts allowed parties to choose whatever law suited them best at any given time it could lead to confusion and uncertainty as well as unfairness when one party is unaware or unable to understand all potential legal implications associated with a particular choice.