The NIE (numero identificacion extranjero) is a fiscal identity number for all who wish to carry out any transactions in Spain. This number is also your identification number when registering as a resident on the foreigners register. The number is either included in a white A4 certificate or on the green coloured A4 residents certificate. These numbers are for life and the white certificate becomes obsolete once you have residency as the number is transferred to the new certificate.
NIE Validity. Sensibly the administrative offices have gone back to the original system and providing the certificate with no expiry date. Those who acquired a NIE under the previous but not original system, will find that it states that the certificate expires in three months. This is only the certificate and not the number.