This report presents a linear model for friction-induced vibration. The solutions to this model might be unstable in spite of a constant coefficient of friction. This is in agreement with the fact that squeal occurs not only just before stops. The method has been used to analyze squeal in both drum and disc brakes.The drum brake model consists of a drum and a shoe, the shoe being flexible while the drum is stiff. The analysis shows that instability may occur in the shoe-drum contact even with a stiff drum. Thus the shoe might be the “motor” of the vibration that propagates to the other parts of the brake system. The mode shapes for these unstable solutions occur as non-synchronous wave motions rather than as simple harmonic motions. These wave motions correspond well to measured motions of squealing brake shoes.The disc brake analysis shows similar instability, i.e. unstable non-synchronous wave motions with a flexible pad and a stiff disc. The correspondence with measured motions is good in this case, too.