One of the key issues concerning the durability of composites is the strength and stiffness degradation during service. Traditionally, these materials have been analyzed by methods which do not take into account variations in the material at the fiber/matrix level. In addition, manufacturing techniques have advanced enough so that composites can be designed from the fiber/matrix level up. As a result, it is important to predict the effect microlevel variations in the material have on macroscopic behavior. Therefore, it is vital to use a micromechanics model to calculate stress and displacement variations. In this study, the strength and stiffness of polymer matrix composites will be determined. To accomplish this, a variational model which calculates microstresses and strains due to damage is used in conjunction with a statistical strength model to predict strength. The results are compared to experimental results of uniaxial strength of carbon fiber composites. In addition, the stiffness of a continuous fiber composite was predicted and compared to a rule of mixtures equation of stiffness. A comparison showed very good agreement. To study the effect of damage, the stiffness of a continuous fiber composite with fiber fragmentation is predicted as a function of fragmentation length and fiber volume fraction. Finally the stiffness of a short-fiber composite is predicted and compared to analytical and experimental results.