The NIE Number is a tax identification number in Spain, known in Spanish as the "Número de Identificación de Extranjero".
The NIE Number is issued by the National Police of Spain and in accordance to Spanish law is required for the purchase or sale of any real estate, vehicle, or boat within Spain.
Once issued an NIE number, it will never change, irrelevant of marital status, name changes or residence status. The NIE Number is a national number, and therefore the location of application is irrelevant as the number can be used throughout the whole of Spain.
Although largely ignored by the vast majority of foreigners up until the year 2005, the National Tax Identification number of Spain has been a requirement for all individuals conducting any type of business within Spain that involves purchasing or selling any items.