A review is made of various information concerning wave groups, or sequence of high waves among ocean waves. The distribution of the number of consecutive high
waves is derived by the random theory ,which serves as the asymptote for waves with spectra of broad bandwidth. Ocean waves show a tendency of slightly longer lengths of runs than the theory due to the existence of a weak dependency of successive wave heights.
　For waves with spectra of narrow bandwidth, the length of wave groups can be calculated by the theory of wave envelopes. The expression of the mean length of wave groups is presented with a formula of correction for discrete counting of the number of high waves in a wave group. Theories are compared with the field and simulation data. The data of the distribution of the length of conditional runs that contain the highest waves in respective wave records are presented.
　Discussions are made on the possible applications of wave group analysis, especially for the phenomena characterized with nonlinearity in wave amplitudes or those with hysteresis of wave actions.