Assignment of liability is a type of contractual assignment whereby the liability arising from breach of obligation in the principal contract is transferred to a third party through a tripartite agreement among the original obligor (assignor), the obligee, and the third party (assignee). The contingent and accessory nature of this contract renders its existence and continuation contingent upon the validity of the principal contract. The legal effect of this institution is the transfer of contractual liability to the third party in the event of non-performance of the obligation, which in fact constitutes a form of guarantee for compensation of the obligee's damages. Therefore, the nullity, rescission, or termination of the principal contract will result in the extinction of the assignment of liability contract as well. In civil law systems, the validity of this contract can be justified by invoking the principle of freedom of will and the rule of agency, except in cases of express statutory prohibition; whereas in the common law system, due to statutory limitations on the acceptance of types of assignments and their restriction to assignment of rights, this contract is deemed to lack legal validity. In the Iranian legal system, liability insurance and performance bonds may be considered as instances of assignment of liability. The present research, employing a descriptive analytical method, examines the fundamental questions of how the legal validity of the assignment of liability contract is evaluated and what are the conditions of validity, scope, and legal effects thereof.