The mean and low-frequency horizontal wave drift forces on 2 types of semi-submersible structures in regular and in irregular waves are determined from model tests and calculations. For the measurement of the low-frequency drift forces in irregular waves use is made of a special dynamic system of restraint. Comparison of measured and computed drift forces in irregular waves show increasing divergence between predictions based on 3-dimensional potential theory and results of experiments with increasing severity of the irregular sea conditions. Comparison between computed and measured mean drift forces in regular waves show increasing divergence at lower wave frequencies. A simple model for approximating viscous contributions to the drift forces in irregular waves is applied to some test results and it is shown that the correlation between measurements and predictions is improved.In order to gain more detailed insight in the mechanisms of the viscous contribution to the drift force tests were carried out with single fixed vertical cylinder in regular waves. The results of tests confirm that in conditions of waves without current the major part of the viscous contribution to the drift force is confined to the splash zone of the cylinder.