Squealing brakes could be a symptom of several conditions including broken anti-rattle clips, worn rotors, worn pads, lack of pad insulation, or incorrect rotor surface cut or no surface cut.   New brakes will sometimes squeal if the metal fibers on the brake pad are too close to one another or are not seated properly.  While it can be frustrating, the squeaking will go away when the pads wear beyond this point.  Your mechanic will do an inspection and let you know the real cause of the sound.