Negative thermal expansion was found for ZrW2O8 from 0.3 kelvin to its decomposition temperature of about 1050 kelvin. Both neutron and x-ray diffraction data were used to solve and refine the structure of this compound at various temperatures. Cubic symmetry persists for ZrW2O8 over its entire stability range. Thus, the negative thermal expansion behavior is isotropic, Essentially the same behavior was found for isostructural HfW2O8. No other materials are known to exhibit such behavior over such a broad temperature range. These materials are finding applications as components in composites in order to reduce the composites’ overall thermal expansion to near zero.