This depends on your nationality.

No Visa is required (for tourists) for visits of up to 90 days if you are a citizen of ehe European Union, Norway, Iceland, Australia, New Zealand, Canada, Switzerland or the United States as well as several other countries.

If you are unsure, check with your Spanish Consulate/Embassy well before your flight day.

Your passport or identity card needs to be valid for a minimum of 3 months (and 6 months for certain countries).