Many marine renewable energy (MRE) conversion systems including wave, floating wind, Ocean Thermal Energy Conversion (OTEC) and some tidal energy devices, are moored in place. The choice of mooring system is critical as it directly affects installation, energy take-off, and long term reliability, and hence has a significant influence on costs. Installation has been estimated to account for 27% of lifetime cost for a tidal turbine. This is an area where the other marine industries, notably offshore oil and gas, have extensive experience, but the high energy regions in which MRE devices are deployed pose particular installation difficulties and operating conditions. There has been a strong movement towards replacing steel with synthetic fiber ropes offshore in recent years, particularly in deep water off Brazil and in the Gulf of Mexico . Lightweight materials can simplify handling and reduce vessel and crane size but this is not the prime mover towards synthetic fiber moorings for marine energy. The main arguments are the possibility to adapt the mooring to the large movements of floating devices, using rope compliance to reduce peak loads while minimizing energy loss, and reduced cost. This requires both a detailed knowledge of the material options and design tools which can optimize stiffness, strength, damping and long term behavior. The large range of fibers and rope constructions available offers extensive possibilities for tailoring the mooring to the response of the device and maximizing energy recovery.