Schengen basics
EU/EEA citizens travel freely with just an ID card or passport. Many other nationalities (including UK, US, Canada, Australia) can enter visa-free for short stays, generally capped at 90 days within a rolling 180-day period across the whole Schengen Area, not just Greece.
Who needs a Schengen visa
Nationalities without a visa-free agreement need to apply for a Schengen visa in advance through a Greek embassy or consulate, or the embassy of another Schengen country if applicable. Processing can take a few weeks, so this is not a last-minute step.
ETIAS note
Visa-exempt non-EU travelers should check whether the EU's ETIAS travel authorization applies at the time of their trip — this is a separate, simple pre-registration, not a full visa, but it is a genuine step some travelers overlook.
If you already hold a valid Schengen visa or are from a visa-exempt country, Athens is straightforward to enter — just track your 90/180-day Schengen allowance if you're combining it with other European stops.