The viscoelastic properties of short-fiber composites are complex and not well-understood. Previous experimental work has shown that the viscoelastic properties of short-fiber composites are affected by both elastic fibers and the matrix, which is baffling since elastic fibers do not exhibit any time-dependence of their own. The goal of this study was to understand why and how elastic fibers can alter time-dependent behavior when contained in a composite. In this thesis, conventional shear-lag theory was adapted to include a time-dependent matrix and a novel analytical model was used to predict the tensile relaxation modulus of short-fiber composites. The model highlighted the importance of incorporating both the time-dependent tensile modulus of the matrix as well as its time-dependent shear modulus. Investigations using the model showed that since stress transfer in a short-fiber composite occurs through interfacial shearing, the time-dependent shear modulus of the matrix results in time-varying stress transfer the fiber. Since the stress in the fiber is time-dependent, it exhibits an apparent stress relaxation stemming from the relaxing shear modulus of the matrix. The model predictions were validated using finite-element simulations and experimental data. Comparison to real data confirmed the hypothesis that the time-dependency observed in elastic fibers stemmed from the indirect time-dependency imposed by the time-varying stress transfer from the matrix. The model was also used to determine the effect of various parameters including fiber aspect ratio and fiber volume fraction. For the first time, a critical aspect ratio for viscoelasticity was introduced. This was defined as the aspect ratio at which the contribution to composite stress relaxation by the fiber is maximized. The effect of fiber orientation was also examined, and an analytical model was developed to predict the stress relaxation of composites containing randomly-oriented fibers. It was found that random orientation in the plane would shrink the effect of fibers by one-third of what would be observed in oriented composites. In the last part of the thesis, we investigated the strain rate-dependence of short fiber-reinforced foams. The study highlighted a potential area where knowledge of the stress relaxation behavior of the short-fiber composites could prove useful.