After a piece of rubber undergoes a mechanical distortion, it is never the same again. The truth of this statement depends, of course, on the degree of precision and discrimination of the observations. Superficially, and for most most practical purposes, it may appear to be unaltered. The point is that in any deformation of rubber there are always some irreversible processes which lead to the conversion of mechanical energy into heat and to a departure from the behavior of an ideal elastic material. Tests which have been used to evaluate hysteresis loss in rubberlike materials may be classified as follows : 1. Low-speed stress-strain loop. 2. Impact resilience. 3. Free vibration. 4. Forced vibration at resonance. 5. Forced vibration ; nonresonance. We shall first discuss the general principles involved in these different methods of testing and then will describe the experimental procedures and results in more detail. For relatively large, slow deformations, depending on the experimental conditions, imperfections in elasticity become evident as hysteresis, permanent set, creep, time lag for recovery or stress relaxation.