This thesis describes a study of the dynamic behaviour of natural rubber both under dynamic extension and retraction, in which the dominant feature is the propagation of an extensional or unloading wave of short duration in the specimen. For retraction, both a high-speed photographic and an optical method of recording have been used to observe the propagation of the wave. Finite waves propagate without dispersion in the specimen for retractions from extensions below 250%, while at higher extensions the wave disperses as it propagates.