The recent development of clay-polymer nanocomposite materials has led to an increased interest in the structure and properties of clay minerals. In this thesis the reactivity of the clay mineral montmorillonite is explored by means of density functional theory based calculations. In particular three aspects are considered: catalytic properties, cation migration and dehydroxylation. The origin of the catalytic properties of the clay mineral is investigated in the context of the synthesis of clay-polymer nanocomposite materials, by in sttu, intercalative polymerisation. It is found that catalysis is most likely to occur at the clay mineral lattice-edge where exposed aluminium atoms act as Lewis acid sites. Migration of lithium cations into the clay mineral lattice is explored by means of first principles molecular dynamics. Comparison of calculated hvdrox-vl stretching frequencies, with those from experiment indicates that cations migrate to vacant octahedral sites, as oppose to the ditrigonal cavities. Dehydroxylation of the clay mineral is examined by consideration of a cis-vacant pyrophyllite structure. It is shown that dehydroxylation leads to formation of a tyan8-vacant structure, with aluminium in trigonal bipyramidal coordination and a highly distorted tetrahedral layer. Differences in the dehydroxylation behaviour of cm and tran8-vacant pyrophyllite are shown to be due to the fact that in the former adjacent hydroxyl groups bridge different pairs of aluminium atoms, while in the latter they are both bonded to the same pair. Overall density functional theory based calculations are shown to be a powerful tool for the studly of the structure and reactivity of clay minerals.