Degradation of composite materials in marine environments has been investigated experimentally and with analytical and numerical methods. Basic mechanical properties, fiber volume fraction, moisture absorption curves and transverse tensile properties after water absorption were determined. Transverse fracture surfaces of dry and wet composites were inspected in a scanning electron microscope (SEM). In addition, the edge replication technique was applied. Micromechanical stress analysis of a composite subjected to mechanical, thermal and moisture loading were performed using analytical methods and finite elements. Transverse stiffness and stress levels for interfacial debonding and matrix failure were calculated and correlated with transverse stiffness and strength obtained experimentally.