There is much current interest in the electrical properties of CaCu3Ti4O12, CCTO, ceramics, since they are, depending on their method of fabrication, heterogeneous materials with semiconducting grain cores and insulating grain or domain boundaries. This unusual electrical microstructure gives rise to a very high effective permittivity associated with the geometric dimensions of the insulating regions and the possibility of applications in electroceramics as, for instance, a novel capacitor material. Scientific interest in CCTO continues, in particular to gain understanding of the reasons for the electrical heterogeneity and to modify its properties by compositional control or doping. This proposal aims to clarify four basic features of the materials science of CCTO: first its true stoichiometry since there is considerable evidence that it is either non-stoichiometric or exists over a range of cation compositions; second, its high temperature stability and in particular, its melting behaviour and whether or not this is preceded by loss of Cu2O from the crystal lattice; third, its intrinsic electrical properties as a function of temperature and atmosphere, and in particular, the nature of the transition from activated hopping to localised electron motion with decreasing temperature and the dependence of this phenomenon on composition; fourth, the factors that control its ceramic microstructure and in particular, the occurrence of anomalous grain growth leading to a duplex structure on the firing of certain samples. Given the continuing great interest in the properties of CCTO, this fundamental material science study is essential to provide underpinning information to allow the fabrication of CCTO in various forms and to optimise control of its properties.