If you are like most people, you will have to transfer at some airport, unless you fly point-to-point. Here is the info you need to know:

Are you flying inside the US? If yes, you pass security at some airport and you will stay in the sterile area as long as you do not have to exit to get luggage, change airline or change airline alliances or change buildings somewhere.

Are you flying from outside Schengen Europe to Schengen Europe (any airport) and connecting to an airport which is outside the Schengen area (see country names at left)?

Example 1: You fly from Washington Dulles, to Frankfurt and then to London. If yes, the security you did in Washington will remain valid all the way to FRankfurt at which point you redo security for the London flight, if you transit in a German airport (you do security to enter the Schengen area for those countries that offer mutual security).

Example 2: You fly from Dublin or London to Frankfurt and then to Hamburg. You will do security in Dublin or London (valid for the first flight) and then in Frankfurt do security again to enter the Schengen sterile area and no more security will be required as long as you fly via the Schengen underlined countries shown in the sidebar at left.

Are you flying from outside Europe to a Schengen area airport and then to another Schengen area airport (see countries in the sidebar)?

If yes, you will do security at the first Schengen airport for that one flight.

Are you flying from outside Schengen Europe to an underlined Schengen area airport and transiting to another underlined Schengen country airport?

If yes, you will so security at the first Schengen country airport and it will remain valid for all other flights to these or via these underlined country airports.

Are you flying from a Schengen country (underlined at left) and then fly through another airport located in an underlined country at left)? You will do security once at the beginning.

Example 1: You arrive in Munich airport and then you transit to Copenhagen. You will do security in Munich and then you will enter the sterile area. You will arrive in Copenhagen in the sterile area. You will be able to fly from Copenhagen without doing security again.

Example 2: You arrive to Milan from outside Europe. You will do security at Milan and enter the sterile area. You will be able to fly to Brussels or Amsterdam or Stockholm and at these airports again take additional flights with no more security.