This paper will present a brief overview of the present understanding of radiation effects in materials. Progress in fundamental understanding of void swelling, phase formation and stability, irradiation creep, and post-irradiation mechanical properties has been gained through comparison of experimental data with the predictions of theoretical models. Applications of the understanding of radiation effects in materials have led to the development of candidate structural materials for use in aggressive radiation environments. Compositional and microstructural controls have been used to achieve the necessary properties and radiation damage resistance. Some of the key issues facing pressure vessel steels, ceramics, and fusion structural materials are described.