All foreign employees working for Spanish companies and self-employed foreigners in Spain must usually contribute to Spanish social security. Generally if you're an employee in Spain, you will be insured under Spanish social security legislation and won't have any liability for social security contributions in your home country or country of domicile.

However, social security agreements exist between Spain and over 40 countries, including all EU countries and the USA, whereby expatriates may remain members of their home country's social security scheme for a limited period.

EU nationals transferred to Spain by an employer in their home country can continue to pay social security abroad for one year (form E-101 is required), which can be extended for another year in unforeseen circumstances (when form E-102 is needed). However, after working in Spain for two years, EU nationals must contribute to the Spanish social security system.

If you're working in Spain, your employer will usually complete the necessary formalities to ensure that you're covered by social security. If he doesn't do it, you must obtain an attestation that you're employed in Spain and register at the nearest social security office to your home. Your local town hall will give you the address of your local office or it will be listed under Seguridad Social in your local Yellow Pages. You will need to produce passports and (certified) birth certificates for all dependants and a marriage certificate (if applicable). You may also need to provide copies with official translations (but check first as translations may be unnecessary).  you will also need proof of residence such as a property deed of sale (escritura) or a rental contract.
After you have registered you will receive a registration card (cartilla de la seguridad social or tarjeta sanitaria), usually by post around four to eight weeks later. A married couple with one partner working are covered by the same social security card and number, as are all dependants (e.g. children under 16) who are listed on your social security card.

More Information
Instituto Nacional de la Seguridad Social,
Subdirección General de Relaciones Internacionales,
Padre Damian, 4, Madrid 28036. Tel: (91) 564 7681.
Free telephone information is available on 900-166565 (mainland and the Balearics), 900-103535 (Las Palmas) and 900-103838 (Tenerife).