This work compares different analytical and numerical methods to evaluate the dynamic response of Spar platforms due to unidirectional and multidirectional waves, current and wind. Various analytical models of wave kinematics and hydrodynamic forces as well as different methods of numerical analysis are compared. Of particular interest are the second order difference frequency forces and structural responses. Some numerical predictions in time domain using Morison’s equation and second order diffraction theory are compared to measured lab and field data. The statistical nature of the response is also studied.