This chapter presents a description of these major categories of nuclear reactor structural materials. Structural materials can be shown as a group listed in seven main categories into which the primary components and materials of nuclear fission reactors are classified. The structural materials of nuclear reactors provide the physical containment, mechanical strength, and structural support for reactor components. Structural materials comprise metals and alloys, ceramics, and cermets. Aluminum has been and still continues to be used as a cladding and structural material for teaching and research reactors. It can result in distortion of fuel assemblies and core structural components. Stainless steels are attractive as structural material in fast reactors, thermal reactors, radioactive wastes, radioisotopes, and other nuclear applications. An important effect of neutron irradiation on the low-carbon structural steels in reactor pressure vessels is a shift in the transition temperature at which the material goes from ductile to brittle and a reduction in upper-shelf energy.