Abstract: Irradiation damage in materials for nuclear reactors, particularly for fusion reactors, is a serious problem. For example, the pressure vessel in a fission power plant becomes brittle after exposure to neutron irradiation for many years. In the case of fusion reactors, in addition to the increase in ductile-to-brittle transition temperature, irradiation-induced swelling occurs in structural materials that need to tolerate high-dose irradiation of several hundreds of displacements per atom (dpa). The irradiation of particles (such as neutrons, ions, and electrons) with high energy introduces a large number of point defects, i.e., self-interstitial atoms and vacancies, into materials. Such point defects aggregate together to form self-interstitial atom clusters as interstitial loop and vacancy clusters as void, stacking fault tetrahedra, or vacancy loop. Then, these clusters affect the microstructure and properties of materials. Moreover, these clusters play a more important role than individual point defects during the irradiation damage process. Even after research for decades, many questions about clusters remain unanswered partially because of the difficulties in irradiation test and cluster observation. This review paper explained the structures of clusters and the effect of clusters on irradiation damage in materials. As a unique research of this author’s group, the formation of vacancy-type dislocation loops with sizes of up to 100 nm in iron was introduced, including the effect of hydrogen and its isotope and the effect of alloy elements on the formation of vacancy-type dislocation loops. There are two different kinds of vacancy-type dislocation loops, i.e., those having a Burgers vector of b =<100> and those having a Burgers vector of b =1/2<111>. The density of the first type is approximately one order of magnitude higher than that of the second type. The one-dimensional motion of self-interstitial atom clusters and the research activities in this field were also discussed in detail, and the one-dimensional motion would be a key factor effecting the irradiation damage of high entropy alloys.