An investigation into fatigue damage mechanisms in carbon fiber reinforced polymer (CFRP) composites under biaxial loading has been conducted. The goal is to capture early stage damage and obtain an improved understanding of the physics of failure under such complex loading. An optimization technique is used to design composite cruciforms and static biaxial load tests and Digital Image Correlation (DIC) are performed to validate the design. The stiffness degradation is measured to monitor damage progression under constant amplitude biaxial fatigue loading. The results show that damage initiates in surface plies as surface fibers transversely separate, and propagates in subsurface layers, causing delamination. Medium and high cycle biaxial fatigue failure occurs in subsurface plies because of stress redistribution due to property degradation in surface plies, while low cycle biaxial fatigue failure occurs in surface plies due to the rapid progression of damage.