The fatigue behaviour of fibre-reinforced polymer composites is typically characterised by costly and time-consuming stress–life testing. This thesis explores an alternative, energy-based approach by adapting complex modulus-based linear viscoelastic theory to describe hysteresis energy dissipation in continuous fibre composites under tensile–tensile fatigue. A modified framework introducing energy-based R–ratio corrections enabled consistent definitions of a viscoelastic loss factor, cyclic dissipation ratios and critical dissipated energy (CDE) across stress levels and load ratios. Analysis of both a literature reference and experimentally tested laminates showed that effective loss factors correlated with Dynamic Mechanical Analysis (DMA) results. The intersection with pristine DMA values provided reliable estimates of high-cycle fatigue strength (HCFS), consistent with existing infrared thermography methods. CDE was identified as a robust fatigue failure criterion, independent of R in the intermediate to high-cycle regime. These findings highlight the potential of energy-based methods to reduce testing requirements while improving predictive capability in composite fatigue.