This dissertation presents theoretical and experimental studies on the phenomenon of frictionally excited thermoelastic instability or TEI in automotive disk brakes and clutches. In this dissertation the finite element method is used to investigate the effect of geometry on TEI problems. Results are obtained for two-dimensional layer and three-dimensional geometries. The hot spots are generally focal in shape, though modes with several reversals through the width start to become dominant at small axial wavenumbers. The dominant wavelength and critical speed are not greatly affected by the three-dimensional effects, except for banding modes. Also, the three-dimensional solutions can be approximated as a monotonic interpolation between the...