The large volume uses of rubber involve its cushioning, quieting, vibration-energy absorbing and isolating characteristics. Applications such as tires, belts, and engine mounts employ rubber under conditions of rapidly repeated deformations which are relatively small compared to ultimate breaking values. Although ultimate values and fatigue tests are important as criteria of quality, they are of no assistance to the engineer, designer or technologist for anticipating the response of his product to the forces experienced in service. To fill this need, there has grown up a body of technical and scientific methods, data, and principles usually referred to as the field of dynamic properties. The term dynamic properties as applied to elastomers usually means vibration properties which are displayed in response to sinusoidal forces. But restriction to sinusoidal forces is unduly narrow. In principle, both theory and experiment can be extended to more complex force patterns, such as square or saw tooth waves, if there is occasion to do so. The distinctive feature of the field of dynamic properties lies, perhaps, more in the amplitudes of the deformations, which are relatively small, than in the force diagram or the frequency. Results at quite low frequencies are sometimes included in work regarded to be on dynamic properties but the amplitudes are always rather small. In polymer science, the field of dynamic properties has tended to be restricted to amplitudes so low that the properties are essentially linear. This would provide a convenient definition but it is an unacceptable limitation from a technological standpoint where it is highly desirable to extend the work into the nonlinear region. On the other hand, fatigue and failure phenomena are definitely excluded from consideration here. Ordinary tensile stress-strain tests have a dynamic aspect but such results are not included in the catagory of dynamic properties. The field to be known as the dynamic properties of elastomers logically comprises the properties exhibited in response to periodic or transient forces for all types of deformation small enough not to induce appreciable fatigue or failure during the investigation. There is no good reason for limiting the frequency on either the low or high side. Temperature is, of course, a most important variable.