The operating environment to be experienced by the nuclear components of a fusion reactor is unique and leads to a number of new phenomena and effects. New experimental knowledge is necessary to resolve many of fusion's remaining issues. Investigation of the required experiments reveals the importance of simulating multiple interactions among physical elements of components and combined effects of a number of operating environmental conditions. Some experiments require neutrons not only as a source of radiation damage effects but as a practical economical means for bulk heating and producing specific nuclear reactions. The evaluation of required facilities suggests important conclusions. Present fission reactors and accelerator-based neutron sources are useful and their use should be maximized worldwide, but they have serious limitations. Obtaining adequate data for fusion nuclear technology over the next 15 years requires a number of new nonneutron test facilities in addition to the use of fission reactors. Experiments in the fusion environment will then be required for integrated tests and concept verification. The key nuclear needs for a fusion facility are 20 MW of deuterium-tritium fusion neutron power over 10 m2 of experimental surface area with long (<1000 s) plasma burn and 2 to 10 MW · yr/m2 fluence capability. Fusion test devices with fusion power >100 MW are shown to be undesirable because of high cost and high risk. The analysis favors fusion devices that are able to operate at low total power and high power density. For fusion devices with large minimum power, e.g., conventional tokamaks, results indicate strong incentives for two separate test devices: one for plasma physics experiments and the other for fusion engineering research experiments.