The Interlibrary Loan Service provides for the needs of its users missing documents in the Library funds from other libraries in the country and abroad. The library sends by post library documents to other libraries on request. A standard form, provided with complete and accurate bibliographic data, is used. All types of documents and copies of: deposit documents, reference books, periodicals (only individual pages or articles), archival materials, unique, rare and valuable publications can be ordered through the ILS.
In international interlibrary lending, the library works with IFLA's voucher system.

The costs of sending and receiving documents on ILS and IILS are at the expense of the guarantor. Right to use the services of ILS and IILS have users of the National Library, who have valid consumer cards and have a permanent address in the city of Plovdiv.

The documents are only used in the reading rooms of the Library for a period of 1 month.

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