This paper presents a working guide to methods and associated data for estimating loading on offshore structures due to waves and currents. Its primary concern is with existing practice in which the formula known as J. R. Morison's equation is used extensively, but some attention is given to diffraction theory methods adopted for large monolithic types of structure and a section on the influence of marine roughness on loading is included. Regarding Morison's equation, a comprehensive review of published data on the relevant coefficients is presented. The considerable uncertainties which still exist in some areas are stressed but advice is offered on the best values to be used in the light of current knowledge. Other important questions discussed include the selection of appropriate wave theories and associated particle kinematics, the effects on fluid loading of proximity and inclination of structural members and the special problems of estimating impact of "slamming" forces.