For connecting domestic flights, you almost never have to exit and reenter security, though there are some exceptions at airports where the terminals aren't all connected. For domestic-to-international connection, it's still pretty rare that you have to exit and reenter security, even if you're changing terminals.

Whether you need to go through security for a connecting flight depends entirely on your point of origin and destination. For domestic-to-domestic connections in the U.S. or within the EU (Schengen Area), you generally stay "airside" and do not need to clear security again, provided you don't exit the terminal. However, for international-to-domestic connections (e.g., flying from London to NYC and then to Chicago), you must clear customs and re-enter security at your first port of entry. This is because you must collect your luggage and clear immigration before entering the domestic terminal. In 2026, some major hubs like London Heathrow or Delhi Airport may require security checks even for international-to-international transfers if you are moving between certain terminals. Always check your boarding pass; if it requires a terminal change that involves a shuttle bus outside the secure area, a re-screening is almost certain.