Whether you need a visa for a connecting flight depends on the country of transit, your nationality, and the length of your stay in the airport. Many countries offer a "Transit Without Visa" (TWOV) policy if you stay in the international transit area and your connection is under 24 hours. However, some countries, like the United States and Canada, require almost all passengers to clear customs and immigration even for a connection, meaning you would need a transit visa or an Electronic Travel Authorization (eTA/ESTA). In 2026, the rollout of the ETIAS in Europe and the ETA in the UK means that even "visa-free" travelers must have digital authorization to transit through major hubs like Heathrow or Frankfurt. Additionally, if you have to change airports (e.g., from Heathrow to Gatwick) or if you have a "self-transfer" where you must collect and re-check bags, you are legally "entering" the country and will almost certainly need a full visa or entry permit. Always check the specific "Transit Visa" requirements for your transit hub at least a month before you fly to avoid being denied boarding.