Damping materials dissipate mechanical energy when cyclically strained by converting it to heat. For sinusoidal strain the amount of energy dissipated per cycle depends on temperature, frequency, and amplitude. Each of these should be measured and reported. If the energy dissipated is proportional to the square of the amplitude, the damping mechanism is called linear and the damping may be reported as energy per cycle loss as a function of amplitude, storage, and loss modulus, or storage modulus and loss ratio. The measurement of system energy loss may be made by direct measurement of force, deflection, and the phase angle between them. The energy loss may also be inferred from the rate of decay in free vibration, the electrical impedance of or the energy input to a vibrating electromechanical system of which the specimen is a part. Regardless of the method used, it is impossible to distinguish between the energy dissipation in the specimen and that dissipated elsewhere in the system. This latter must either be accurately known or negligibly small. In order properly to interpret the data, the state and distribution of strain must be known though the volume of the specimen. Methods for measuring and controlling each of these parameters will be discussed and illustrated.