Nuclear factors affecting the effectiveness of control rod materials for power reactors with a thermal or near thermal neutron spectrum are surveyed. The principal elements and isotopes which qualify as neutron absorbers are compared and physical requirements such as rod thickness, surface density, and weight ratios are stated. The importance of epithermal absorption and the nuclear effects of burnup due to long neutron exposure in power reactors are shown. The characteristics of absorption isotope chains such as europium are described and finally, the deleterious effects of cladding and scattering cross section of poison diluents on control effectiveness are shown.