All known materials exhibit rheologic or time-dependent behavior to some degree under loading. For individual materials, the relative importance of this behavior is usually determined by the conditions of use. With steel, for example, time-dependent behavior may be unimportant at normal temperatures and quite important at elevated temperatures. With soils and the materials normally used in constructing high quality flexible pavements, time-dependent behavior under load is quite important over the normal conditions of use. In dynamic loadings of these pavement materials, it can be sufficient to approach design problems from a phenomenological viewpoint, that is, while desirable, it is not essential to rationalize why the material responds as it does, either at the molecular or the particle level. In such an approach, however, it may be necessary to assume that the material is homogenous and isotropic so that the response of any part in any direction is the same as that of the whole. While untrue at the micro-volume level this is not an unusual working assumption and, for particulate materials such as soils and asphaltic concretes, it necessarily involves measurements that are statistical averages in which the dimensions of the largest components are small in comparison with the total dimensions of the unit under study. With these considerations the time response of these materials to dynamic loadings can be examined.