In this paper, theories on the establishment of the content of the foreign law are analyzed. These theories besides enjoying legal support in the jurisprudence of countries such as France, are defendable in private international law systems of other countries including Iranian legal system. The first of these theories considers the ascertainment of the content of the foreign law among the duties of the parties.
According to the second theory the courts must attempt to seek that content but its establishment and knowledge thereof are not their obligation. The third theory invokes the legal nature of the cases and the Parties' will in the choice of the applicable law. In some cases the Parties may invoke non-applicability of the foreign law rather than its applicability. As to the application of the peremptory (jus cogens) rules of the conflict of laws, however, the judge is bound to apply the foreign appropriate law and to attempt to seek it and attain its content. And finally, in the fourth theory the search for the content of the foreign law is necessarily the obligation of domestic courts the non-fulfillment of which is considered a violation of the rules of the conflict of laws.