An investigation of stress history and dynamic disturbance effects on the strength and stiffness of chesionless soils has been conducted. Extensive laboratory and field tests were performed in the research program. Laboratory tests performed in a device for measuring the elastic shear modulus under laterally constrained conditions revealed that it was a function of the soil compressibility as well as the previous stress conditions. A new model for predicting it under zero lateral strain was formulated. Decreases in the elastic shear modulus of cohesionless soil samples were observed following a disturbance even though densification occurred. Time dependent increases in the modulus prior to disturbance were partially or completely destroyed following shear strains greater than approximately 0.02%. The magnitude of the decrease was primarily a function of the shear strain amplitude and was relatively independent of the void ratio, confining stress, number of shear strain cycles, and shear strain rate. For samples subjected to shear strains less than approximately 0.1%, the decrease in the modulus was temporary; however, for shear strains greater than 0.1%, the modulus never regained the values measured prior to the disturbance. In the field, decreases in tip resistance, local friction, and shear wave velocity were observed following the detonation of an explosive in a partially saturated, medium-dense sand. Virtually no change in the soil density of the dilatometer horizontal stress index was recognized. Tests were performed at locations where the estimated shear strains from the blast ranged from 3 - 0.003%. Time dependent increases in the tip resistance and local frictio following the blast were observed; however, at locations close to the blast the increases were not significant enough to compensate for the initial decrease. Time dependent changes in the shear wave velocity following the blast were inconclusive. Decreases in the field shear wave velocity due to a disturbance were greater than decreases measured in a laboratory tested sample.