If you are from a member-country of the Schengen agreement - Austria, Belgium, the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, plus Iceland and Norway (which are not EU members)- you do not need a passport to enter Spain. An official national ID card will suffice.
Citizens from EU countries that do not form part of the Schengen agreement must carry a valid passport to enter Spain. These are the UK and Ireland, as well as new members of the EU that do not yet participate in Schengen are Cyprus (joined in 2004) and Bulgaria and Romania (joined in 2007).
If you are from any of the aforementioned countries, Switzerland or member-countries of the European Economic Area (Norway, Iceland & Liechtenstein), you do not need a special visa to enter or reside in Spain.
Citizens of the following countries do not need a visa to stay in Spain for less than 90 days, only a valid passport: Andorra, Argentina, Australia, Austria, Belgium, Bolivia, Brazil, Brunei, Canada, Chile, Costa Rica, Croatia, El Salvador, Guatemala, Honduras, Hong Kong & Macao (China), Israel, Japan, Malaysia, Mexico, Monaco, New Zealand, Nicaragua, Panama, Paraguay, Singapore, South Korea, United States, Uruguay, Venezuela.
If you require visa to enter Spain, you can find more information here: https://www.schengenvisainfo.com/spain-visa/
Disclaimer: The information above is intended to assist conference attendees with their travel arrangements. Each individual traveling to the conference is responsible for researching all requirements necessary for entry to Spain specific to his or her case. ESWI has no influence on an individual’s application to a Spanish Embassy or any other agency for a visa to travel to Spain and cannot be held responsible for the outcome of an individual’s visa application.