The effect of interphases on the mechanical behavior of fiber-reinforced composites is studied. The interphase is modeled as distributed continuous shear and normal springs. Two problems, a fiber pulled out of a matrix and a cracked fiber in a matrix, are discussed. An analytical solution is developed in terms of simultaneous integral equations. An asymptotic analysis reveals singularities in the stress and strain functions. Numerical results are obtained to study the effect on the micromechanical behavior of a composite as a function of relative mechanical properties of the fiber, the matrix and interphase regions, and the ratio of the interphase thickness to the fiber width.