The paper presents an analysis of the mooring system effects on the dynamics of an arbitrary floating wave energy converter (WEC) and on the efficiency of the device. The mooring system dynamics are calculated by a finite differences method, which accounts for the lines’ physical characteristics such as mass and stiffness, as well as for the hydrodynamic inertial and damping characteristics. A method is proposed and applied to consider the inertial, damping and stiffness effects of the mooring system on the WEC’s linear dynamics. An identification method is used to determine the linear mooring system coefficients from pre-calculated nonlinear time domain forces of the mooring system on the floater (the importance of nonlinear effects is assessed as well). Finally the mooring system dynamics is represented by additional inertial, damping and stiffness matrices which are added to the ones of the WEC. The influence of the mooring system on the WEC dynamics and efficiency is assessed in terms of wave induced motions and absorbed power. Results are presented for the transfer functions, statistics in selected stationary sea states and the expected annual absorbed energy. A wave scatter diagram representative of the Portuguese Pilot Zone is used for the annual predictions.