The effect of interphases on the mechanical behavior of fiber-reinforced composites is studied. Two problems, a cracked fiber in a matrix, and a fiber pulled out of a matrix, are discussed. The interphase is modeled as continuous shear and normal springs. 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 behaviour of a composite as a function of relative mechanical properties of the fiber, the matrix and the interphase regions, and the ratio of the interphase thickness to the fiber radius.