An analytical model for stiffness degradation of composite laminates with damage under static or cyclic loading is proposed. For static loading, two different modelling approaches have been created. Both account for shear-induced microscopic matrix damage and matrix cracking within each ply orientation of a laminate. In one approach the residual transverse tensile and shear moduli of a ply are determined by equating the energy density of the undamaged ply to that of the ply with damage, for a same applied load. Predictions excellent agreement with test results for cross-ply laminates, while stiffness degradation is for shear dominated layups. The other static approach applies energy equivalence only to the transverse tensile behavior of a ply. For shear, it predicts the residual shear modulus by accounting for the creation of permanent shear strains. Agreement with test results is excellent for cross-ply laminates, and excellent to good for shear dominated layups. The physical foundation behind this model is not as rigorous as the previous one, and as such needs more work. The proposed fatigue model assumes the matrix strength of a ply to be randomly distributed. Kassapoglou’s residual strength model for fatigue loading is used to express fatigue life as a function of matrix strength. The static model is then used to estimate stiffness degradation as a function the same matrix strength. Connecting the two, stiffness degradation as a function of fatigue cycles is obtained. Validation was performed on cross-ply laminates and agreement with results is excellent.