Resilient modulus and Young's modulus are parameters increasingly used to characterize the behavior of pavement materials, both in the laboratory and in the field. The small-strain modulus response of unbound aggregate base coarse materials to various moisture environments was documented. Modulus is not constant, even when held at constant moisture, and significant changes in modulus occur with drying and wetting of the material. The response to drying and wetting cycles appears to be repeatable and suggests that the underlying mechanism that controls the response is reversible. This behavior can have a significant effect on the use of modulus for pavement design and quality acceptance.