Although the absorption of hydrogen by palladium has been studied for almost a century, interest in this system has not diminished. On the contrary, with the current realisation of the important role of nonstoichiometry in solid reactions, this system is receiving even more attention as an excellent example of a compound which exhibits a large degree of nonstoichiometry. Lewis (I) has recently reviewed the hydrogen-palladium system in some detail and readers should refer to his series of articles for a general background. This article will be restricted to the key role that diffraction techniques have played in the understanding of this system.