New battery. New alternator. Still doesn't start.

Do you hear a single click from under the hood every time you try to start it? If so, you may have a bad starter. Ignition switch is a possible problem as well. Is there an aftermarket alarm system that may be causing problems? My next test would be to test for battery voltage at the starter while someone is trying to start the car. If there is full voltage that doesn't go away while trying to start the car, you will probably need a starter.