[en] Zirconium hydride is an example of an Einstein crystal in which the motion of the hydrogen can be treated to first order as an isotropic harmonic oscillator. In 1936 Fermi first described neutron interactions with such an oscillator. The first measurements of neutron interactions with zirconium hydride, or with any metal hydrides, were made as part of the early program to develop the TRIGA reactor. After the first measurements in 1957 demonstrated the existence of a bound hydrogen level with sharply defined energy, a broad range of neutron measurements were conducted over the next 10 years. Ultimately, the tests were able to demonstrate that four bound levels exist for the hydrogen. Very high resolution measurements have also been able to demonstrate the presence of forces due to second or third nearest neighbor interactions. At this point, the behavior of the hydrogen as moderator in zirconium hydride is sufficiently well understood for the purposes of reactor theory. Additional measurements and theoretical developments are still required for a complete understanding of the role of hydrogen or deuterium in a metal hydride. (author)