Long term behavior of polymeric geogrids is one of the major concerns in designing reinforced slopes and walls. It is commonly considered that the strength and stiffness of the geogrids decrease due to aging and environmental deterioration. In addition to laboratory creep test results, the aging under field conditions and environmental deterioration of polymer maerials can be effectively studied by analyzing case histories. Tensile strains developed in geogrids were measured in reinforced cohesive soil slopes in an instrumented test embankment, 12 m high with 1:1 side slopes. The field measurements of the geogrid strains, for three years during construction and for four years thereafter, indicate that the time-dependent tensile strains in the geogrids were insignificant.