Initiating a lawsuit does not inherently require elaborate formalities. However, for the sake of legal order, harmonization of laws, and the protection of individual rights, the enactment of procedural rules governing various aspects of litigation is deemed essential. One such crucial aspect pertains to the procedural method of filing a claim before competent judicial authorities. Thus, initiating legal proceedings must conform both to a defined format and to the observance of the specific procedural requirements associated with that format. In civil litigation, the filing of a complaint is the formal mechanism for bringing a claim, and such a complaint is subject to a host of mandatory legal provisions, notably those relating to the plaintiff's articulation of the claim. The specification of the claim, as a fundamental element of the lawsuit, is approached differently in the legal systems of Iran and the United States. Nevertheless, in both jurisdictions, the requirements apply equally to ancillary proceedings such as third-party impleader actions, which are recognized as innovative legal mechanisms in both systems. This article seeks to address the following question: How is the claim specified in third-party impleader actions, and what procedural requirements govern this process? The hypothesis underpinning this research is that although third-party impleader actions are considered ancillary rather than principal claims, the procedural rules applicable to principal claims are likewise mandatory in such actions. The methodology employed in this study is interpretive and content-analytical, relying on library-based legal sources. The findings indicate that all procedural and substantive requirements governing the filing of a principal claim must also be strictly observed in third-party impleader actions. In fact, given that impleader actions are particularly susceptible to procedural irregularities, a heightened degree of legal precision is required to safeguard procedural integrity.