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 re-clear security depends on your specific itinerary. For domestic-to-domestic connections (e.g., NYC to Atlanta to Dallas), you generally stay "airside" and do not go through security again. However, if you are arriving on an international flight and connecting to a domestic one (e.g., London to JFK to Chicago), you must clear customs, collect your bags, and go through TSA security again. For international-to-international transfers, most major hubs (like London Heathrow or Dubai) have "transit security" checkpoints where you must be screened again before entering the departure hall for your next flight. Always check the airport's layout, as some connections between different terminals may require you to exit the secure area and re-enter through the main security lanes.