Depends on the country and your passport. Many hubs allow airside transit without a visa; the US, Canada and Australia require an ESTA/eTA/ETA even to transit; the UK requires a transit visa for some nationalities. Check before you book a connection.
No visa needed to stay airside (most nationalities): Singapore, Doha, Dubai, Istanbul, Hong Kong, Seoul, Tokyo, Frankfurt, Amsterdam (non-Schengen to non-Schengen), Zurich.
Authorisation required even to transit: the US (ESTA or visa — no sterile transit), Canada (eTA for most visa-exempt nationals, with some transit exemptions), Australia (transit visa or ETA for many), New Zealand (NZeTA).
UK: some nationalities need a Direct Airside Transit Visa; many visa-free visitors now need an ETA to enter.
Schengen: connecting from a non-Schengen to a Schengen flight means entering the Schengen area — you need the right to enter.
Always check the airline’s Timatic result or the government site for your specific passport and routing.
Related questions
How long does my passport need to be valid to travel?
Many countries require six months' validity beyond your arrival or departure date; the Schengen area requires three months beyond your planned exit and a passport issued within the last ten years. Airlines enforce these rules at check-in. Full answer →
Can I leave the airport during a layover?
Often yes, if you can legally enter the country, your bags are checked through, and you have enough time — usually six hours or more. Subtract immigration, transit both ways and a 90-minute return buffer before deciding. Full answer →
Airline, airport and government rules change without notice. Treat this as a starting point and confirm with the airline or authority for your specific trip. Request a correction if something is out of date.