The major structure type of breakwaters in Japan is a mixed type which consists of a rubble mound foundation and an upright section. Mixed type breakwaters of which the upright sections are covered with wave dissipating concrete blocks are also popular in Japan. In the paper, the hydraulic characteristics of those conventional breakwaters are first described including already established formulae. In particular, experimental data on the transformation of irregular waves transmitted behind a breakwater during the propagation are newly presented.
　On the other hand, changes of surrounding conditions related to breakwater constructions are recently emerging. In particular, necessity of constructing breakwaters in deep and rough seas results in the marked increase of the cost, and demands the development of new economical breakwater structures. In the paper, several new breakwater structures such as sloping top caissons, multi-scelluar, caissons, curved slit caissons, wave power extracting caissons. trapezoidal caissons and dual cylinder caissons are introduced with the present states of their developments.