Abstract: A method and an apparatus for checking the authenticity of documents, such as banknotes or credit-cards. The documents comprise a number of randomly distributed conductive fibres of which the distribution is scanned by microwaves and the response is transformed into a digital coded signal. A digital mark on the document, which is representative for the distribution of an individual document, is read off, transcoded, and compared with said coded signal for producing an approval signal. The use of a microwave scanning system in conjunction with documents having a random distribution of suitable fibres ensures a great repetitivity and security of the system.