Fiber–matrix interface debonding initiated from random fiber breaks is known to be one of the key damage mechanisms in unidirectional (UD) composites subjected to quasi-static and cyclic (fatigue) loading. Growth of fiber–matrix interface debonds leads to stiffness reduction and eventually to the final failure of the UD composite through coalescence of multiple debond cracks. This chapter overviews the current state of the art in modeling fiber–matrix interface debonding in UD composites. The methods reviewed in this chapter are based on fracture mechanics principles of the energy release rate. Analytical models for steady-state debond growth are presented. Finite-element method (FEM) based models for analyzing the growth of short debonds and the effects of edges and neighboring fibers are also presented.