Design of a seawall requires the estimation of the rate of wave overtopping with due consideration for wave irregularity. The present report proposes the calculation of the expected rate of irregular wave overtopping with the information of wave height distribution and the overtopping rate by regular trains of waves. The validity of calculation has been confirmed with laboratory tests with irregular trains of waves. Diagrams for the estimation of expected rate of wave overtopping have been prepared for seawalls with vertical faces and those with protective mounds of artificial blocks. The diagrams are based on the Rayleigh distribution of wave heights and various laboratory data on regular wave overtopping. An application of the diagrams to the cases of seawalls damaged by wave overtopping has revealed the existence of the threshold overtopping rates for structural damage. Analysis of natural drainage of overtopped water through orifices at the front wall is also presented with several recommendations for the design of drainage system.