Conference attendees from outside the European Economic Area (EEA) may require a travel visa to enter the Schengen Area. If you require a letter of invitation to support your visa application, you have the opportunity to indicate this during the registration process. A letter of invitation will be sent to you shortly after registering. If you want the hard copy of the letter, the shipment has a cost of about 35€, which will be added to your registration fee.
Consult the list of non-EEA countries whose nationals do not require a visa. In some cases, a biometric passport (ICAO compliant) is required to enter without a visa (see footnotes for your territory). Contact the Spanish embassy or consulate for your territory when in doubt.
Short stay visa for professional reasons: application guide.