Damage accumulation in Glass Fiber Reinforced Polymer (GFRP) composites is monitored based on Poisson's ratio measurements for three different fiber stacking sequences subjected to both quasi-static and quasi-static cyclic tensile loadings. The sensor systems utilized include a dual-extensometer, a biaxial strain gage and a novel embedded-biaxial Fiber Bragg Grating (FBG) sensor. These sensors are used concurrently to measure biaxial strain whereby the evolution of Poisson's ratio as a function of the applied axial strain is evaluated. It is observed that each sensor system indicates a non-constant Poisson's ratio, which is a sign of damage accumulation under the applied tensile loading. As the number of off-axis plies increases, transverse strain indicates a notable deviation from linearity due to the formation of transverse cracking, thereby leading to a larger reduction in Poisson's ratio as a function of applied axial strain. Here, it is demonstrated that biaxially embedded FBG sensors are reliable to monitor the evolution of Poisson's ratio, unlike biaxial strain gages which record strain values that can be significantly influenced by the cracks formed on the surface of the specimen. (C) 2016 Elsevier Ltd. All rights reserved.