Many modes of damage can be observed in composite materials, including matrix cracks, ﬁber breakage, ﬁber-matrix de-bonding, and so on. Much work has been done trying to quantify each of these damage modes, their evolution with respect to load, strain, time, number of cycles, etc., and their eﬀect on stiﬀness, remaining life, etc. Continuum Damage Mechanics (CDM) represents all these failure modes by the eﬀect they have on the mesoscale behavior (lamina level) of the material. That is, CDM calculates the degraded moduli of the laminas and laminate in terms of continuum damage variables. Then, either strength or fracture mechanics failure criteria are used to detect damage initiation. Finally, damage evolution is predicted in terms of empirical hardening equations set up in terms of additional parameters, such as the hardening exponent used for metal plasticity. For example, a form of CDM is used in Chapter 10 to represent the degradation of the interface between laminas.