In contrast to other ancillary suits (such as competing claims, compulsory presence of a third party, and intervention of a third party), there are no particular regulations in the Iranian Civil Procedural Law to define principles and rules govering “additional claim”. The word “additional” within the context of this term has two meanings. Its general meaning includes any change in the essential elements of the claim, the purpose of the claim, and the reason for the claim, which generally known as “changes in the claim”. In its specific meaning, it refers to a claim submitted under a new demand which is attached to the original claim due to its relation to the original claim. Accordingly, this paper analyses the concept, necessity, conditions and rules govering the additional claim in order to clarify the existing ambiguous points, differences and similarities with the changes in the claim.