In judicial practice, it is increasingly common for a court to find it necessary to apply the law of a State other than the one in which the proceedings are taking place, either because the parties have agreed to do so - usually by including a choice of law clause in a contract - or by applying the so-called "conflict rules", which determine which substantive law the courts of each country have to apply.

However, unlike the law of their own State, which the courts must know and apply by virtue of the principle of iura novit curia, the courts are not obliged to know or apply foreign law, and it is up to the parties to prove it so that the judges can apply it.

The proof of foreign law is regulated in Spain - in a brief, not to say incomplete manner - in Law 29/2015, of 30 July, on international legal cooperation in civil matters (henceforth, LCJIC) and in the Law of Civil Procedure (henceforth, LEC). These regulations establish that their content and validity must be accredited, aspects to which jurisprudence usually adds interpretation and applicability to the case (for all, STS 17 April 2015).

With regard to the means of proof of foreign law, the LEC and the LCJIC do not establish how it must be accredited. Our jurisprudence has established that the courts are not bound by the evidence and data provided by the parties and must assess the evidence presented to them in accordance with the "rules of healthy criticism", a criterion that is specified in Article 33.4 LCJIC when it states that "no report or opinion, whether national or international, on foreign law shall be binding on the Spanish courts". However, the courts have been admitting, among others, the presentation of reports issued by public authorities (whether from the foreign state in question or the Subdirectorate General of International Legal Cooperation of the Spanish Ministry of Justice) or by experts in the foreign law to be proved (affidavits), means which, in our opinion, continue to be the most appropriate.

Although there is case law that admits the presentation of this evidence in the second instance or even in cassation, the most prudent thing is to provide it together with the complaint or response. Like any other document, if it is written in a language that is not official in the place where the court is located, it must be translated.

The judicial body that hears the proceedings may collaborate in the evidence of foreign law but is not obliged to do so. To this end, the LCJIC establishes a mechanism so that Spanish judicial bodies can process requests for information on foreign law through the Ministry of Justice, and Spain is party to several international bilateral and multilateral agreements that facilitate this work, such as the European Convention on Information on Foreign Law made in London on 7 June 1968.

Finally, it should be noted that the SC has repeatedly stated that if foreign law is not proved, Spanish law will be applied, a thesis criticised by several authors as being contrary to the principle of the imperative nature of conflict rules. The current LCJIC reinforces this criterion in Article 33.3, although it qualifies that such application will take place "exceptionally".