There are many causes for brakes to squeal. We will cover the three most common in this article.
The number one cause of brake squealing is worn brakes. The brake pads of today come with a small piece of metal that hangs off the side of the pad which is designed to squeal whenever your brakes are thin enough to need replacement. It is a nice design feature that allows you to change your pads before any damage occurs to the brake rotor.
The second most common cause of brake squealing is poorly designed cheap brake pads. These "economy priced" pads are made of cheaper material that creates more heat and vibrates across the rotor causing a loud squealing noise.
The third most common cause of brake squealing is glazed brake rotors. When braking, the brake system will build up immense amounts of heat that cause the surface of the brake rotor to become very slick. This slick surface will cause even the best of brake pads to make noise. With today's rotors being cast as thinly as they are the best solution is replacement. In previous years brake rotors were designed thicker allowing room for resurfacing the brake rotors to cure the squealing condition.