A small-scale field experiment was conducted on wave forces on upright breakwaters, and the following observations were made. The largest waves in a sea state yield one of two very characteristic time histories of the horizontal force, , according to whether or not there is overtopping. With a large overtopping, has a very sharp peak at the instant at which water reaches beyond the top of the wall. Without overtopping, has a smooth history with its maximum when the water reaches the highest elevation on the wall. The sharp peaks of the wave-force per unit of length at a given section of a breakwater are estimated to have a negligible effect on the force on one caisson, provided that the wave crest makes an angle of only a few degrees with the breakwater. The dimensionless pressure distributions are not very sensitive to the ratio between the of the incident waves and the elevation of the wall above the mean water level. Our field experiment confirms that Goda’s model (GM) is very effective for estimating the positive wave pressures, although it is slightly conservative. However, Goda’s model yields some negative wave forces that prove to be smaller than the largest negative forces measured in the course of the experiment. Boccotti’s model (BM), based on a linear theory with virtual wave heights, proves to be very effective for estimating negative wave pressures.