As the range of applications for fiber-reinforced polymer (FRP) composite materials in civil engineering constantly increases, there is more and more concern with regard to their performance in critical environments. The fire behavior of composite materials is especially important since complex physical and chemical processes such as the glass transition and decomposition occur when these materials are subjected to elevated and high temperatures, possibly leading to considerable loss of stiffness and strength. This stiffness and strength degradation in composite materials under elevated and high temperatures is the result of changes in polymer molecular structures. When polyester thermosets are subjected to elevated and high temperatures, they undergo three transitions (glass transition, leathery-to-rubbery transition, and rubbery-to-decomposed transition), corresponding to four different states (glassy, leathery, rubbery and decomposed). At a certain temperature, a composite material can therefore be considered as a mixture of materials that are in different states. As the content of each state varies with temperature, the composite material exhibits temperature-dependent properties. Since these changes in state can be described using kinetic theory, the quantity of material in each state can be estimated and the thermophysical and thermomechanical properties of the mixture can thus be determined. These concepts formed a basis for the development of thermophysical and thermomechanical property sub-models for composites at elevated and high temperatures and even for the description of post-fire status. Incorporating these thermophysical property sub-models into a heat transfer governing equation, thermal responses were calculated using a finite difference method. Integrating the thermomechanical property sub-models within structural theory, the mechanical responses were described using a finite element method and the time-to-failure was also predicted by defining a failure criterion. The modeling results for temperature responses, mechanical responses and post-fire behavior were compared with those obtained from structural endurance experiments on full-scale cellular GFRP (glass fiber-reinforced polymer, in this case polyester resin) panels subjected to a four-point bending configuration and fire from one side. The modeling results for time-to-failure were compared with those from the experiments carried out on GFRP tubes under combined compressive and thermal loadings. In each experimental setup, two different thermal boundary conditions were investigated – with and without water cooling through specimen cells – and good agreement was found. The understanding gained and modeling of the behavior of GFRP composites under elevated and high temperatures carried out in this thesis could be applicable for different composite materials, and also benefit investigations regarding both active and passive fire protection techniques in order to improve the fire resistance of structures made of such materials.