Simulations using density functional theory were carried out to investigate the defect properties of zirconium diboride (ZrB(2)) and also the solution and diffusion of He and Li. Schottky and Frenkel intrinsic defect processes were all high energy as were mechanisms giving rise to nonstoichiometry; this has implications for high-temperature performance. Li and He species, formed by the transmutation of a (10)B, should therefore mostly be accommodated at the resulting vacant B sites or interstitial sites. Because Li is considerably more stable at the vacant B sites, He will be accommodated interstitially. Furthermore, He was found to diffuse as an interstitial species through the lattice with a low activation energy. This would be consistent with He being lost from the ZrB(2) but with Li being retained to a much greater extent.