A fatigue model for fiber-reinforced polymeric matrix composites is proposed. This model uses a substructuring approach to predict the behavior of composite laminates based on an integration of simpler composite elements. These simpler composite elements are known as “elemental blocks” and are two-layer laminates such as [0/90] or [0/45]. These laminates are chosen in order to capture the influence of the constraint between laminae on the fatigue performance. Tension-tension fatigue tests were conducted on a number of elemental block geometries, and the modulus degradation for each of the geometries was measured and described using simple functions. The results showed that the damage accumulation and concomitant reduction in stiffness varied systematically from geometry to geometry. Using these results, a simple fatigue model is proposed to predict the behavior of more complex composites by combining the degradation functions for the individual elemental blocks. This model is based on classical lamination theory and uses the measured stiffness reduction for each elemental block, as a function of the number of cycles, to predict the overall composite stiffness matrix. In order to test the model predictions, fatigue tests were conducted on more complex composites consisting of a number of elemental blocks, and the accumulation of damage was measured again using compliance and strain gages attached to the specimen surface. These results were compared with model predictions and reasonable agreement was obtained between the measured and predicted stiffness degradation.