The causes of turbine blade excitation are described, and the factors that influence amplitudes of vibration are discussed. Blade rotation in an asymmetrical pressure field produces a periodic force on the blade. Harmonics of this periodic force excite the blades at their natural frequencies. Gas disturbances created by nozzle vanes also can excite the blades. Amplitudes are affected by the type of exhaust system combined with the type of turbine housing, turbine housing design, engine speed, vane design, and any local gas disturbance or condition that creates asymmetrical gas flow through the turbine housing.