Legalisation certifies a document's (diplomas, certificates) formal conformity with the law binding in the place of its issue and the authenticity of the seals and signatures placed on the document. If your country is not a party to the Apostille Convention, your documents will need to be legalised. The first step of the procedure is to have the document legalised by an appropriate office of your country (usually either the educational authorities or branches of the ministry of foreign affairs, and sometimes by consuls of your country in Poland), and then by a Polish consul, which completes the process.
