Schengen, briefly
Spain is part of the Schengen Area, a group of European countries with no internal border checks between them. For visa purposes, what matters is your total time spent across the entire Schengen Area, not just Spain — a trip that also includes France or Italy counts against the same limit.
The 90/180 rule
Visa-exempt visitors (many nationalities, including the US, UK, Canada, and Australia among others) can typically stay in the Schengen Area for up to 90 days within any 180-day period without a visa. This is a rolling window, not a simple calendar reset — the 180 days looks backward from any given date, which trips people up if they've made multiple short European trips in the same year.
ETIAS
ETIAS is a pre-travel authorization system for visa-exempt visitors, expected to apply to entries into the Schengen Area — it functions similarly to the US ESTA system: an online application completed before travel, not a visa in the traditional sense. Implementation timelines have shifted more than once; check the current official status before your specific trip rather than assuming last year's requirements still apply.
Who needs an actual visa
Nationalities not covered by a visa-waiver arrangement need to apply for a Schengen visa in advance through a Spanish consulate or visa center in their home country — this process can take weeks, so it's not something to leave until close to travel dates if it applies to you.
What to actually carry
A passport valid for at least the planned duration of stay (many countries require 3–6 months' additional validity beyond your travel dates specifically), and any visa or authorization documentation if applicable. Border officials occasionally ask for proof of onward travel or accommodation — rare, but worth having accessible digitally just in case.