A model for predicting the buckling and wrinkling behavior of glass-fiber-reinforced polymer (GFRP) plates and sandwich panels subjected to in-plane biaxial compression-tension loading is presented in this paper. In-plane biaxial compression-tension loading occurs in the webs of plate girders or cell-core sandwiches. The model is able to simulate two counteracting effects of increasing transverse tension load on buckling and wrinkling loads as observed in experiments. A stabilizing effect tends to push the plate back to the median plane and thereby delays the onset of buckling/wrinkling instability. In contrast, lateral contraction accelerates the bending of the plate, which leads to a significant decrease in buckling/wrinkling loads. In composite plates, the first effect predominates and increases the buckling loads, while in sandwich panels the second effect is dominant and decreases the wrinkling loads. The theoretical predictions are in good agreement with the corresponding experimental results. DOI: 10.1061/(ASCE)CC.1943-5614.0000277. (C) 2012 American Society of Civil Engineers.